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Date and Times

Click on the tab buttons below for each section

Create Date and Times

Create Date and Times

today()
#> [1] "2020-11-10"
now()
#> [1] "2020-11-10 00:27:59 SAST"

From Strings

Dates

ymd("2017-01-31")
#> [1] "2017-01-31"
mdy("january 31st, 2017")
#> [1] "2017-01-31"
dmy("31-Jan-2017")
#> [1] "2017-01-31"
ymd(20170131)
#> [1] "2017-01-31"

Date Times

ymd_hms("2017-01-31 20:11:59")
#> [1] "2017-01-31 20:11:59 UTC"
mdy_hm("01/31/2017 08:01")
#> [1] "2017-01-31 08:01:00 UTC"
# force a dttm creation by supplying a tz
ymd(20170131, tz = "UTC")
#> [1] "2017-01-31 UTC"
dmy(31012017, tz = "Africa/Johannesburg")
#> [1] "2017-01-31 SAST"

From Individual Components

Sometimes date and time components are scattered over multiple columns in your data. This is the case in the flights data.

You can use make_date() and make_datetime() to create a date or dttm field.

flights %>% 
  select(year:day, hour, minute)
#> # A tibble: 336,776 x 5
#>     year month   day  hour minute
#>      
#>  1  2013     1     1     5     15
#>  2  2013     1     1     5     29
#>  3  2013     1     1     5     40
#>  4  2013     1     1     5     45
#>  5  2013     1     1     6      0
#>  6  2013     1     1     5     58
#>  7  2013     1     1     6      0
#>  8  2013     1     1     6      0
#>  9  2013     1     1     6      0
#> 10  2013     1     1     6      0
#> # ... with 336,766 more rows

flights %>%
  select(year:day, hour, minute) %>%
  mutate(
    departure = make_datetime(year = year, month = month, day = day,
                              hour = hour, min = minute)
  )
#> # A tibble: 336,776 x 6
#>     year month   day  hour minute departure          
#>                   
#>  1  2013     1     1     5     15 2013-01-01 05:15:00
#>  2  2013     1     1     5     29 2013-01-01 05:29:00
#>  3  2013     1     1     5     40 2013-01-01 05:40:00
#>  4  2013     1     1     5     45 2013-01-01 05:45:00
#>  5  2013     1     1     6      0 2013-01-01 06:00:00
#>  6  2013     1     1     5     58 2013-01-01 05:58:00
#>  7  2013     1     1     6      0 2013-01-01 06:00:00
#>  8  2013     1     1     6      0 2013-01-01 06:00:00
#>  9  2013     1     1     6      0 2013-01-01 06:00:00
#> 10  2013     1     1     6      0 2013-01-01 06:00:00
#> # ... with 336,766 more rows
make_dttm <- function(year, month, day, time){
  # this func will make a proper dttm for the dep_time,
  # sched_dep_time etc.
  make_datetime(year, month, day,
                # divide by 100 and get the integer part - hour part
                time %/% 100,
                # divide by 100 and get the remainder - minutes part
                time %% 100
                )
}

(flights_dt <- flights %>%
  filter(!is.na(dep_time), !is.na(arr_time)) %>%
  mutate(
    dep_time = make_dttm(year, month, day, dep_time),
    sched_dep_time = make_dttm(year, month, day, sched_dep_time),
    arr_time = make_dttm(year, month, day, arr_time),
    sched_arr_time = make_dttm(year, month, day, sched_arr_time)
  ) %>%
  select(origin, dest, ends_with("delay"), ends_with("time")))
#> # A tibble: 328,063 x 9
#>    origin dest  dep_delay arr_delay dep_time            sched_dep_time     
#>                                           
#>  1 EWR    IAH           2        11 2013-01-01 05:17:00 2013-01-01 05:15:00
#>  2 LGA    IAH           4        20 2013-01-01 05:33:00 2013-01-01 05:29:00
#>  3 JFK    MIA           2        33 2013-01-01 05:42:00 2013-01-01 05:40:00
#>  4 JFK    BQN          -1       -18 2013-01-01 05:44:00 2013-01-01 05:45:00
#>  5 LGA    ATL          -6       -25 2013-01-01 05:54:00 2013-01-01 06:00:00
#>  6 EWR    ORD          -4        12 2013-01-01 05:54:00 2013-01-01 05:58:00
#>  7 EWR    FLL          -5        19 2013-01-01 05:55:00 2013-01-01 06:00:00
#>  8 LGA    IAD          -3       -14 2013-01-01 05:57:00 2013-01-01 06:00:00
#>  9 JFK    MCO          -3        -8 2013-01-01 05:57:00 2013-01-01 06:00:00
#> 10 LGA    ORD          -2         8 2013-01-01 05:58:00 2013-01-01 06:00:00
#> # ... with 328,053 more rows, and 3 more variables: arr_time ,
#> #   sched_arr_time , air_time

flights_dt %>%
  ggplot(aes(dep_time)) +
  # how's the distribution across the year looking?
  geom_freqpoly(binwidth = 24*60*60) # binwidth = 1 day

flights_dt %>% 
  filter(dep_time < ymd(20130102)) %>%
  ggplot(aes(dep_time)) +
  # how's the distribution of dep times across a day looking
  geom_freqpoly(binwidth = 15 * 60) # how's it looking every 15 min

Note that when you use date-times in a numeric context (like in a histogram), 1 means 1 second, so a binwidth of 86400 means one day. For dates, 1 means 1 day.

From other types

You may want to switch between a date-time and a date. That’s the job of as_datetime() and as_date().

as_datetime(today()) # today() returns a date, convert to dttm
#> [1] "2020-11-10 UTC"
as_date(now()) # now() returns a dttm, convert to date
#> [1] "2020-11-10"

Sometimes you’ll get date/times as numeric offsets from the “Unix Epoch”, 1970-01-01. If the offset is in seconds, use as_datetime(); if it’s in days, use as_date().

as_datetime(60 * 60 * 10) # add 10 hours to default dttm 1970-01-01 UTC
#> [1] "1970-01-01 10:00:00 UTC"
as_datetime(0) # default dttm
#> [1] "1970-01-01 UTC"
as_date(365 * 10 + 2) # add 10 years (365 * 10) + 2 days to get to 1980-01-01
#> [1] "1980-01-01"
as_date(365 * 10) # add 10 years - 365 days per year * 10
#> [1] "1979-12-30"
as_date(0) # default date (add 0 days)
#> [1] "1970-01-01"
as_date(10) # add 10 days to default
#> [1] "1970-01-11"
make_date() # default == Unix Epoch Date
#> [1] "1970-01-01"
make_datetime() # default == Unix Epoch Date
#> [1] "1970-01-01 UTC"

Exercises

  1. What happens if you parse a string that contains invalid dates?

    ymd(c("2010-10-10", "bananas"))
    ymd(c("2010-10-10", "bananas"))
    #> Warning: 1 failed to parse.
    #> [1] "2010-10-10" NA

    You get a warning that some failed to parse.

  2. What does the tzone argument to today() do? Why is it important?

    The today() and now() uses your computer’s time and date. This is usually accurate for your timezone. For example I am in South Africa so you would see SAST when I call now().

    But you may want to do analyses that involve other timezones, or UTC itself. You can use this argument to adjust to the timezone you’re interested in.

    today()
    #> [1] "2020-11-10"
    (utc_today <- today(tzone = "UTC"))
    #> [1] "2020-11-09"
    utc_today == today()
    #> [1] FALSE
    utc_today == today(tzone = "GMT")
    #> [1] TRUE
    now()
    #> [1] "2020-11-10 00:28:05 SAST"
    now("UTC")
    #> [1] "2020-11-09 22:28:05 UTC"
  3. Use the appropriate lubridate function to parse each of the following dates:

    d1 <- "January 1, 2010"
    mdy(d1)
    #> [1] "2010-01-01"
    d2 <- "2015-Mar-07"
    ymd(d2)
    #> [1] "2015-03-07"
    d3 <- "06-Jun-2017"
    dmy(d3)
    #> [1] "2017-06-06"
    d4 <- c("August 19 (2015)", "July 1 (2015)")
    mdy(d4)
    #> [1] "2015-08-19" "2015-07-01"
    d5 <- "12/30/14" # Dec 30, 2014
    mdy(d5)
    #> [1] "2014-12-30"

Date-time components

Date-time components

We will focus on the accessor functions that let you get and set individual components.

Getting components

  • year()
  • month()
  • mday() (day of the month)
  • yday() (day of the year)
  • wday() (day of the week)
  • hour(), minute(), and second()
(datetime <- ymd_hms("2016-07-08 12:34:56"))
#> [1] "2016-07-08 12:34:56 UTC"
year(datetime)
#> [1] 2016

month(datetime)
#> [1] 7

mday(datetime)
#> [1] 8

yday(datetime)
#> [1] 190

wday(datetime)
#> [1] 6

wday(datetime,
     label = TRUE) # abbreviated weekday day instead of number
#> [1] Fri
#> Levels: Sun < Sat

wday(datetime,
     label = TRUE,
     abbr = FALSE) # full weekday name
#> [1] Friday
#> 7 Levels: Sunday < Saturday

hour(datetime)
#> [1] 12

minute(datetime)
#> [1] 34

second(datetime)
#> [1] 56

month(datetime,
      label = TRUE) # abbreviated month name instead of number
#> [1] Jul
#> 12 Levels: Jan < Dec

month(datetime, label = TRUE,
      abbr = FALSE) # full month name instead of number
#> [1] July
#> 12 Levels: January
(current_dt <- now())
#> [1] "2020-11-10 00:28:05 SAST"
year(current_dt)
#> [1] 2020
month(current_dt)
#> [1] 11
mday(current_dt)
#> [1] 10
yday(current_dt)
#> [1] 315
wday(current_dt) # sunday
#> [1] 3
wday(current_dt, 
     label = TRUE) # abbreviated weekday day instead of number
#> [1] Tue
#> Levels: Sun < Mon < Tue < Wed < Thu < Fri < Sat
wday(current_dt, 
     label = TRUE, 
     abbr = FALSE) # full weekday name 
#> [1] Tuesday
#> 7 Levels: Sunday < Monday < Tuesday < Wednesday < Thursday < ... < Saturday
hour(current_dt)
#> [1] 0
minute(current_dt)
#> [1] 28
second(current_dt)
#> [1] 5.663242
month(current_dt, 
      label = TRUE) # abbreviated month name instead of number
#> [1] Nov
#> 12 Levels: Jan < Feb < Mar < Apr < May < Jun < Jul < Aug < Sep < ... < Dec
month(current_dt, label = TRUE, 
      abbr = FALSE) # full month name instead of number
#> [1] November
#> 12 Levels: January < February < March < April < May < June < ... < December
# wday() shows that more flights depart in the week
flights_dt %>% 
  mutate(wday = wday(dep_time, label = TRUE)) %>% 
  ggplot(aes(x = wday)) +
  geom_bar()

# interesting that flights at certain minutes in
# an hour experience lower delays
flights_dt %>% 
  mutate(minute = minute(dep_time)) %>% 
  group_by(minute) %>% 
  summarise(avg_delay = mean(dep_delay, na.rm = TRUE),
            n = n()) %>% 
  ggplot(aes(minute, avg_delay)) +
  geom_line() +
  geom_point(aes(size = n))

# the scheduled departure time, does not show such
(sched_dep <- flights_dt %>% 
  mutate(minute = minute(sched_dep_time)) %>% 
  group_by(minute) %>% 
  summarise(avg_delay = mean(dep_delay, na.rm = TRUE),
            n = n())) 
#> # A tibble: 60 x 3
#>    minute avg_delay     n
#>     <int>     <dbl> <int>
#>  1      0      8.82 59019
#>  2      1     16.6   2085
#>  3      2     12.9    818
#>  4      3     19.8   1381
#>  5      4     16.8   1323
#>  6      5     12.5  13720
#>  7      6     17.8   1345
#>  8      7     14.5   1068
#>  9      8     12.0   1649
#> 10      9     18.5   1411
#> # ... with 50 more rows

sched_dep %>% 
  ggplot(aes(minute, avg_delay)) +
  geom_line() +
  geom_point(aes(size = n))


sched_dep %>% 
  ggplot(aes(minute, n)) +
  geom_line()

You may also round a date to a nearby unit of time, with floor_date(), round_date(), and ceiling_date().

Each function takes a vector of dates to change, and then the name of the unit to round down (floor), round up (ceiling), or round to.

(x <- ymd_hms("2020-08-08 12:01:59.93")) # Saturday 8th Aug
#> [1] "2020-08-08 12:01:59 UTC"

round_date(x, ".5s")
#> [1] "2020-08-08 12:02:00 UTC"

round_date(x, "sec")
#> [1] "2020-08-08 12:02:00 UTC"

round_date(x, "second")
#> [1] "2020-08-08 12:02:00 UTC"

round_date(x, "minute")
#> [1] "2020-08-08 12:02:00 UTC"

round_date(x, "5 mins")
#> [1] "2020-08-08 12:00:00 UTC"

round_date(x, "hour")
#> [1] "2020-08-08 12:00:00 UTC"

round_date(x, "2 hours")
#> [1] "2020-08-08 12:00:00 UTC"

round_date(x, "day") # it's past afternoon so closer to tomorrow
#> [1] "2020-08-09 UTC"

round_date(x, "week")
#> [1] "2020-08-09 UTC"

round_date(x, "month")
#> [1] "2020-08-01 UTC"

round_date(x, "bimonth") # 1,3,5,7,9,11 are the bimonths
#> [1] "2020-09-01 UTC"

round_date(x, "quarter")
#> [1] "2020-07-01 UTC"

round_date(x, "quarter") == round_date(x, "3 months")
#> [1] TRUE

round_date(x, "halfyear")
#> [1] "2020-07-01 UTC"

round_date(x, "year")
#> [1] "2021-01-01 UTC"

round_date(x, "season")
#> [1] "2020-09-01 UTC"
(x <- ymd_hms("2020-08-08 12:01:59.93")) # Saturday 8th Aug
#> [1] "2020-08-08 12:01:59 UTC"

floor_date(x, ".5s")
#> [1] "2020-08-08 12:01:59 UTC"

floor_date(x, "sec")
#> [1] "2020-08-08 12:01:59 UTC"

floor_date(x, "second")
#> [1] "2020-08-08 12:01:59 UTC"

floor_date(x, "minute")
#> [1] "2020-08-08 12:01:00 UTC"

floor_date(x, "5 mins")
#> [1] "2020-08-08 12:00:00 UTC"

floor_date(x, "hour")
#> [1] "2020-08-08 12:00:00 UTC"

floor_date(x, "2 hours")
#> [1] "2020-08-08 12:00:00 UTC"

floor_date(x, "day") # floor of date hence still today even though closer to tomorrow
#> [1] "2020-08-08 UTC"

floor_date(x, "week") # floor so last sunday NOT tomorrow
#> [1] "2020-08-02 UTC"

floor_date(x, "month")
#> [1] "2020-08-01 UTC"

floor_date(x, "bimonth") # 1,3,5,7,9,11 are the bimonths
#> [1] "2020-07-01 UTC"

floor_date(x, "quarter")
#> [1] "2020-07-01 UTC"

floor_date(x, "quarter") == floor_date(x, "3 months")
#> [1] TRUE

floor_date(x, "halfyear")
#> [1] "2020-07-01 UTC"

floor_date(x, "year")
#> [1] "2020-01-01 UTC"

floor_date(x, "season")
#> [1] "2020-06-01 UTC"
(x <- ymd_hms("2020-08-08 12:01:59.93")) # Saturday 8th Aug
#> [1] "2020-08-08 12:01:59 UTC"

ceiling_date(x, ".5s")
#> [1] "2020-08-08 12:02:00 UTC"

ceiling_date(x, "sec")
#> [1] "2020-08-08 12:02:00 UTC"

ceiling_date(x, "second")
#> [1] "2020-08-08 12:02:00 UTC"

ceiling_date(x, "minute")
#> [1] "2020-08-08 12:02:00 UTC"

ceiling_date(x, "5 mins")
#> [1] "2020-08-08 12:05:00 UTC"

ceiling_date(x, "hour")
#> [1] "2020-08-08 13:00:00 UTC"

ceiling_date(x, "2 hours")
#> [1] "2020-08-08 14:00:00 UTC"

ceiling_date(x, "day") # ceiling of date hence tomorrow
#> [1] "2020-08-09 UTC"

# ceiling so tomorrow which is start of new week
# Sunday 9th August 2020
ceiling_date(x, "week")
#> [1] "2020-08-09 UTC"

ceiling_date(x, "month")
#> [1] "2020-09-01 UTC"

ceiling_date(x, "bimonth") # 1,3,5,7,9,11 are the bimonths
#> [1] "2020-09-01 UTC"

ceiling_date(x, "quarter")
#> [1] "2020-10-01 UTC"

ceiling_date(x, "quarter") == ceiling_date(x, "3 months")
#> [1] TRUE

ceiling_date(x, "halfyear")
#> [1] "2021-01-01 UTC"

ceiling_date(x, "year")
#> [1] "2021-01-01 UTC"

ceiling_date(x, "season")
#> [1] "2020-09-01 UTC"

Setting components

You can also use each accessor function to set the components of a date/time. Note that the accessor functions now appear on the left hand side of <-.

(datetime <- ymd_hms("2016-07-08 12:34:56"))
#> [1] "2016-07-08 12:34:56 UTC"

# change year
year(datetime) <- 2020
datetime
#> [1] "2020-07-08 12:34:56 UTC"

# change month
month(datetime) <- 01
datetime
#> [1] "2020-01-08 12:34:56 UTC"

# progress time by 1 hr
hour(datetime) <- hour(datetime) + 1
datetime
#> [1] "2020-01-08 13:34:56 UTC"

Alternatively, rather than modifying in place, you can create a new date-time with update(). This also allows you to set multiple values at once.

update(datetime,
       year = 2020,
       month = 02,
       mday = 02,
       hour = 15)
#> [1] "2020-02-02 15:34:56 UTC"

datetime
#> [1] "2020-01-08 13:34:56 UTC"

# entered a too big date / hour / min?
# these roll over
ymd("2020-02-27") %>%
  update(mday = 29)
#> [1] "2020-02-29"

ymd("2020-02-27") %>%
  update(mday = 30, hour = 22,
         min = 65)
#> [1] "2020-03-01 23:05:00 UTC"
flights_dt %>% 
  # ignore the date, let's make a new date that pulls all observations
  # back to the 1st of Jan
  mutate(dep_hour = update(dep_time, yday = 1)) %>% 
  arrange(desc(sched_dep_time)) %>% 
  head(10) %>% 
  print(width = Inf)
#> # A tibble: 10 x 10
#>    origin dest  dep_delay arr_delay dep_time            sched_dep_time     
#>    <chr>  <chr>     <dbl>     <dbl> <dttm>              <dttm>             
#>  1 JFK    BQN          14         2 2013-12-31 00:13:00 2013-12-31 23:59:00
#>  2 JFK    SJU          19         5 2013-12-31 00:18:00 2013-12-31 23:59:00
#>  3 JFK    SJU          -4       -10 2013-12-31 23:55:00 2013-12-31 23:59:00
#>  4 JFK    PSE          -3        -9 2013-12-31 23:56:00 2013-12-31 23:59:00
#>  5 EWR    SJU          -2         3 2013-12-31 23:28:00 2013-12-31 23:30:00
#>  6 JFK    BOS          15        11 2013-12-31 23:10:00 2013-12-31 22:55:00
#>  7 JFK    SYR          -5         3 2013-12-31 22:45:00 2013-12-31 22:50:00
#>  8 JFK    BUF          31        38 2013-12-31 23:21:00 2013-12-31 22:50:00
#>  9 JFK    PWM         101        96 2013-12-31 00:26:00 2013-12-31 22:45:00
#> 10 JFK    BTV         -10        -4 2013-12-31 22:35:00 2013-12-31 22:45:00
#>    arr_time            sched_arr_time      air_time dep_hour           
#>    <dttm>              <dttm>                 <dbl> <dttm>             
#>  1 2013-12-31 04:39:00 2013-12-31 04:37:00      189 2013-01-01 00:13:00
#>  2 2013-12-31 04:49:00 2013-12-31 04:44:00      192 2013-01-01 00:18:00
#>  3 2013-12-31 04:30:00 2013-12-31 04:40:00      195 2013-01-01 23:55:00
#>  4 2013-12-31 04:36:00 2013-12-31 04:45:00      200 2013-01-01 23:56:00
#>  5 2013-12-31 04:12:00 2013-12-31 04:09:00      198 2013-01-01 23:28:00
#>  6 2013-12-31 00:07:00 2013-12-31 23:56:00       40 2013-01-01 23:10:00
#>  7 2013-12-31 23:59:00 2013-12-31 23:56:00       51 2013-01-01 22:45:00
#>  8 2013-12-31 00:46:00 2013-12-31 00:08:00       66 2013-01-01 23:21:00
#>  9 2013-12-31 01:29:00 2013-12-31 23:53:00       50 2013-01-01 00:26:00
#> 10 2013-12-31 23:51:00 2013-12-31 23:55:00       49 2013-01-01 22:35:00

# let's visualise how many flights leave at different hours in 
# the day by pretending all flights occurred on the same day
# the 1st Jan
flights_dt %>% 
  mutate(dep_hour = update(dep_time, yday = 1)) %>% 
  ggplot(aes(dep_hour)) +
  geom_freqpoly(binwidth = 60 * 5) # every 5 min

Exercises

  1. How does the distribution of flight times within a day change over the course of the year?

    # devtools::install_github("sciencificity/werpals")
    library(werpals)
    flights_dt %>% 
      mutate(month = factor(month(dep_time)),
             dep_hour = update(dep_time, yday = 1)) %>% 
      ggplot(aes(dep_hour, colour = month)) +
      geom_freqpoly(binwidth = 60 * 30) +
      scale_colour_nature(palette = "bryce")

    Feb is lower but it has less days as well.

  2. Compare dep_time, sched_dep_time and dep_delay. Are they consistent? Explain your findings.

    The \(sched\_dep\_time + dep\_delay = dep\_time\). It does for most observations but there is a handful that does not meet this relationship.

    flights %>% 
      select(year, month, day, hour, minute, dep_time, sched_dep_time,
             dep_delay)
    #> # A tibble: 336,776 x 8
    #>     year month   day  hour minute dep_time sched_dep_time dep_delay
    #>    <int> <int> <int> <dbl>  <dbl>    <int>          <int>     <dbl>
    #>  1  2013     1     1     5     15      517            515         2
    #>  2  2013     1     1     5     29      533            529         4
    #>  3  2013     1     1     5     40      542            540         2
    #>  4  2013     1     1     5     45      544            545        -1
    #>  5  2013     1     1     6      0      554            600        -6
    #>  6  2013     1     1     5     58      554            558        -4
    #>  7  2013     1     1     6      0      555            600        -5
    #>  8  2013     1     1     6      0      557            600        -3
    #>  9  2013     1     1     6      0      557            600        -3
    #> 10  2013     1     1     6      0      558            600        -2
    #> # ... with 336,766 more rows
    flights_dt %>% 
      select(dep_time, sched_dep_time,
             dep_delay) %>% 
      mutate(act_dep_time = sched_dep_time + 
               hms(str_glue("0, {dep_delay}, 0"))) %>% 
      filter(dep_time != act_dep_time)
    #> # A tibble: 1,205 x 4
    #>    dep_time            sched_dep_time      dep_delay act_dep_time       
    #>    <dttm>              <dttm>                  <dbl> <dttm>             
    #>  1 2013-01-01 08:48:00 2013-01-01 18:35:00       853 2013-01-02 08:48:00
    #>  2 2013-01-02 00:42:00 2013-01-02 23:59:00        43 2013-01-03 00:42:00
    #>  3 2013-01-02 01:26:00 2013-01-02 22:50:00       156 2013-01-03 01:26:00
    #>  4 2013-01-03 00:32:00 2013-01-03 23:59:00        33 2013-01-04 00:32:00
    #>  5 2013-01-03 00:50:00 2013-01-03 21:45:00       185 2013-01-04 00:50:00
    #>  6 2013-01-03 02:35:00 2013-01-03 23:59:00       156 2013-01-04 02:35:00
    #>  7 2013-01-04 00:25:00 2013-01-04 23:59:00        26 2013-01-05 00:25:00
    #>  8 2013-01-04 01:06:00 2013-01-04 22:45:00       141 2013-01-05 01:06:00
    #>  9 2013-01-05 00:14:00 2013-01-05 23:59:00        15 2013-01-06 00:14:00
    #> 10 2013-01-05 00:37:00 2013-01-05 22:30:00       127 2013-01-06 00:37:00
    #> # ... with 1,195 more rows
  3. Compare air_time with the duration between the departure and arrival. Explain your findings. (Hint: consider the location of the airport.)

    flights_dt %>% 
      select(origin, dest, dep_time, arr_time, air_time) %>% 
      filter(!is.na(air_time), !is.na(dep_time), 
             !is.na(arr_time)) %>% 
      mutate(dest_same_tz = dep_time + 
               hms(str_glue("0, {air_time}, 0"))) %>% 
      filter(arr_time == dest_same_tz)
    #> # A tibble: 196 x 6
    #>    origin dest  dep_time            arr_time            air_time
    #>    <chr>  <chr> <dttm>              <dttm>                 <dbl>
    #>  1 LGA    BNA   2013-01-07 11:50:00 2013-01-07 13:41:00      111
    #>  2 EWR    MSP   2013-01-13 19:47:00 2013-01-13 22:18:00      151
    #>  3 LGA    MDW   2013-01-16 06:51:00 2013-01-16 09:14:00      143
    #>  4 EWR    HOU   2013-01-16 07:07:00 2013-01-16 11:06:00      239
    #>  5 LGA    BNA   2013-01-16 09:43:00 2013-01-16 12:11:00      148
    #>  6 LGA    BNA   2013-01-25 16:19:00 2013-01-25 18:26:00      127
    #>  7 EWR    ORD   2013-01-30 10:16:00 2013-01-30 12:34:00      138
    #>  8 EWR    ORD   2013-01-30 14:19:00 2013-01-30 16:14:00      115
    #>  9 EWR    ORD   2013-01-30 21:55:00 2013-01-30 23:47:00      112
    #> 10 LGA    ORD   2013-10-02 18:53:00 2013-10-02 20:45:00      112
    #> # ... with 186 more rows, and 1 more variable: dest_same_tz <dttm>

    There are only a few where the \(dep\_time + air\_time = arr\_time\). If you look at the dest vs the origin I would have suspected that the airports in the same timezone align but LGA and ORD are different timezones and they align, LGA and BNA also and others too. So truly I am unsure without further digging.

  4. How does the average delay time change over the course of a day? Should you use dep_time or sched_dep_time? Why?

    flights_dt %>% 
      mutate(sched_dep_hr = update(sched_dep_time, yday = 1),
             hr_day = hour(sched_dep_hr)) %>% 
      group_by(hr_day) %>% 
      summarise(avg_delay = mean(dep_delay, na.rm = TRUE),
                n = n()) %>% 
      ggplot(aes(hr_day, avg_delay)) +
      geom_point(aes(size = n)) +
      geom_line()

    I used sched_dep_time since that is when the flight should have left and is a more accurate reflection of how delay time changes over the course of a day.

  5. On what day of the week should you leave if you want to minimise the chance of a delay?

    flights_dt %>% 
      mutate(day_of_week = wday(sched_dep_time, abbr = TRUE, label = TRUE)) %>% 
      group_by(day_of_week) %>% 
      summarise(avg_delay = mean(dep_delay, na.rm = TRUE),
                n = n()) %>% 
      ggplot(aes(day_of_week, avg_delay)) +
      geom_col()

    You best leave on Saturday.

  6. What makes the distribution of diamonds$carat and flights$sched_dep_time similar?

    diamonds %>% 
      count(carat, sort = TRUE)
    #> # A tibble: 273 x 2
    #>    carat     n
    #>    <dbl> <int>
    #>  1  0.3   2604
    #>  2  0.31  2249
    #>  3  1.01  2242
    #>  4  0.7   1981
    #>  5  0.32  1840
    #>  6  1     1558
    #>  7  0.9   1485
    #>  8  0.41  1382
    #>  9  0.4   1299
    #> 10  0.71  1294
    #> # ... with 263 more rows
    
    flights_dt %>% 
      mutate(sched_min = minute(sched_dep_time)) %>% 
      count(sched_min, sort = TRUE)
    #> # A tibble: 60 x 2
    #>    sched_min     n
    #>        <int> <int>
    #>  1         0 59019
    #>  2        30 33104
    #>  3        45 19923
    #>  4        15 18393
    #>  5        55 18347
    #>  6        59 15882
    #>  7        10 14162
    #>  8        25 14057
    #>  9         5 13720
    #> 10        29 13482
    #> # ... with 50 more rows
    
    # out of curiosity how does actual dep_time look?
    flights_dt %>% 
      mutate(min = minute(dep_time)) %>% 
      count(min, sort = TRUE)
    #> # A tibble: 60 x 2
    #>      min     n
    #>    <int> <int>
    #>  1    55 10097
    #>  2    54  9366
    #>  3    56  9161
    #>  4    57  8878
    #>  5    58  8433
    #>  6    53  8378
    #>  7    59  7500
    #>  8    52  7422
    #>  9     0  7160
    #> 10    25  6918
    #> # ... with 50 more rows

    More flights are scheduled at round numbers that we talk about often; quarter past, half past, 6 ’o clock, etc. The actual departure time is much less aligned with how we talk. We don’t usually talk about out flight being 54 minutes past 5! Except when we were kids learning to tell time, then we’re very precise (ask my 6yo who keeps reminding me when I say it’s nearly half past 8 and time for bed, that actually it is only 8:26 so he has 4 more minutes 😆).

    For carats it is similar 0.3, 0.7, 0.4 but there are some anomalies for example 0.32 or 0.31 or even 1.01.

  7. Confirm my hypothesis that the early departures of flights in minutes 20-30 and 50-60 are caused by scheduled flights that leave early. Hint: create a binary variable that tells you whether or not a flight was delayed.

    flights_dt %>% 
      filter(!is.na(dep_delay)) %>% 
      mutate(early = dep_delay < 0,
             dep_min = minute(update(dep_time, yday = 1))) %>% 
      ggplot(aes(dep_min, fill = early)) +
      geom_bar() +
      scale_fill_disney(palette = "pan")

    The flights that leave in minutes 22 - 29 and 49 - 59 do leave earlier than scheduled.

Time spans

Time spans

Now we’re gonna see how arithmetic with dates works and learn about three important classes that represent time spans:

  • durations, which represent an exact number of seconds.
  • periods, which represent human units like weeks and months.
  • intervals, which represent a starting and ending point.

Durations

When we subtract two dates, we get a difftime object.

# How old is Hadley?
(h_age <- today() - ymd(19791014))
#> Time difference of 15003 days

A difftime class object records a time span of

  • seconds,
  • minutes,
  • hours,
  • days, or
  • weeks.

The ambiguity can make difftimes painful to work with, so {lubridate} provides an alternative which always uses seconds: the duration.

as.duration(h_age)
#> [1] "1296259200s (~41.08 years)"

Duration constructors

dseconds(15)
#> [1] "15s"

dminutes(10)
#> [1] "600s (~10 minutes)"

dhours(c(12,24))
#> [1] "43200s (~12 hours)" "86400s (~1 days)"

ddays(0:5)
#> [1] "0s"                "86400s (~1 days)"  "172800s (~2 days)"
#> [4] "259200s (~3 days)" "345600s (~4 days)" "432000s (~5 days)"

dweeks(3)
#> [1] "1814400s (~3 weeks)"

dweeks(3) == 24 * 60 * 60 * 7 * 3 # 24 hrs, 60 min, 60 s in each min, 7 days, 3 weeks
#> [1] TRUE

dyears(1)
#> [1] "31557600s (~1 years)"

Durations always record the time span in seconds. Larger units are converted using standard rules e.g. 60 seconds in a minute, 60 minutes in an hour, 24 hours in day, 7 days in a week, and 365 days in a year. Notice all duration constructors start with d and end with s (i.e. the plural of each time span - ddays)

Duration Arithmetic

You can add and multiply durations.

2 * dyears(1)
#> [1] "63115200s (~2 years)"
dyears(2)
#> [1] "63115200s (~2 years)"

dyears(1)  + dweeks(12) + dhours(15)
#> [1] "38869200s (~1.23 years)"

We can also add and subtract durations to and from dates.

(tomorrow <- today() + ddays(1))
#> [1] "2020-11-11"
(last_yr <- year(today() - dyears(1)))
#> [1] 2019

But durations represent an exact number of seconds, and hence sometimes you might get unexpected results, especially with daylight savings etc. that complicate date time arithmetic.

(one_pm <- ymd_hms("2016-03-12 13:00:00",
                  tz = "America/New_York"))
#> [1] "2016-03-12 13:00:00 EST"
one_pm + ddays(1)
#> [1] "2016-03-13 14:00:00 EDT"

One day after 1pm on March 12, is 2pm on March 13?! 😕 The tz has changed from EST to EDT due to daylight savings time.

Periods

To solve this problem, {lubridate} provides periods. Periods are time spans that work with “human” times, like days and months.

one_pm
#> [1] "2016-03-12 13:00:00 EST"
one_pm + days(1)
#> [1] "2016-03-13 13:00:00 EDT"

Period constructors

seconds(15)
#> [1] "15S"

minutes(10)
#> [1] "10M 0S"

hours(c(12, 24))
#> [1] "12H 0M 0S" "24H 0M 0S"

days(0:5)
#> [1] "0S"          "1d 0H 0M 0S" "2d 0H 0M 0S" "3d 0H 0M 0S" "4d 0H 0M 0S"
#> [6] "5d 0H 0M 0S"

weeks(3)
#> [1] "21d 0H 0M 0S"

years(1)
#> [1] "1y 0m 0d 0H 0M 0S"

months(1:6)
#> [1] "1m 0d 0H 0M 0S" "2m 0d 0H 0M 0S" "3m 0d 0H 0M 0S" "4m 0d 0H 0M 0S"
#> [5] "5m 0d 0H 0M 0S" "6m 0d 0H 0M 0S"

Period Arithmetic

2 * years(1)
#> [1] "2y 0m 0d 0H 0M 0S"
years(2)
#> [1] "2y 0m 0d 0H 0M 0S"

years(1) + weeks(12) + hours(15)
#> [1] "1y 0m 84d 15H 0M 0S"

10 * (months(6) + days(1))
#> [1] "60m 10d 0H 0M 0S"

days(50) + hours(25) + minutes(2)
#> [1] "50d 25H 2M 0S"

Compared to durations, periods are more likely to do what you expect.

# A leap year
ymd(20200101) + dyears(1)
#> [1] "2020-12-31 06:00:00 UTC"
ymd(20200101) + years(1)
#> [1] "2021-01-01"

one_pm
#> [1] "2016-03-12 13:00:00 EST"
one_pm + ddays(1)
#> [1] "2016-03-13 14:00:00 EDT"
one_pm + days(1)
#> [1] "2016-03-13 13:00:00 EDT"

Some planes appear to have arrived at their destination before they departed from NYC.

flights_dt %>% 
  filter(arr_time < dep_time)
#> # A tibble: 10,633 x 9
#>    origin dest  dep_delay arr_delay dep_time            sched_dep_time     
#>    <chr>  <chr>     <dbl>     <dbl> <dttm>              <dttm>             
#>  1 EWR    BQN           9        -4 2013-01-01 19:29:00 2013-01-01 19:20:00
#>  2 JFK    DFW          59        NA 2013-01-01 19:39:00 2013-01-01 18:40:00
#>  3 EWR    TPA          -2         9 2013-01-01 20:58:00 2013-01-01 21:00:00
#>  4 EWR    SJU          -6       -12 2013-01-01 21:02:00 2013-01-01 21:08:00
#>  5 EWR    SFO          11       -14 2013-01-01 21:08:00 2013-01-01 20:57:00
#>  6 LGA    FLL         -10        -2 2013-01-01 21:20:00 2013-01-01 21:30:00
#>  7 EWR    MCO          41        43 2013-01-01 21:21:00 2013-01-01 20:40:00
#>  8 JFK    LAX          -7       -24 2013-01-01 21:28:00 2013-01-01 21:35:00
#>  9 EWR    FLL          49        28 2013-01-01 21:34:00 2013-01-01 20:45:00
#> 10 EWR    FLL          -9       -14 2013-01-01 21:36:00 2013-01-01 21:45:00
#> # ... with 10,623 more rows, and 3 more variables: arr_time <dttm>,
#> #   sched_arr_time <dttm>, air_time <dbl>

They are overnight flights and incorrectly have the same date information for both the departure and the arrival times.

(flights_dt <- flights_dt %>% 
  mutate(overnight = arr_time < dep_time,
         arr_time = arr_time + days(overnight * 1),
         sched_arr_time = sched_arr_time + days(overnight + 1))) %>% 
  filter(overnight == TRUE) %>% 
  print(width = Inf)
#> # A tibble: 10,633 x 10
#>    origin dest  dep_delay arr_delay dep_time            sched_dep_time     
#>    <chr>  <chr>     <dbl>     <dbl> <dttm>              <dttm>             
#>  1 EWR    BQN           9        -4 2013-01-01 19:29:00 2013-01-01 19:20:00
#>  2 JFK    DFW          59        NA 2013-01-01 19:39:00 2013-01-01 18:40:00
#>  3 EWR    TPA          -2         9 2013-01-01 20:58:00 2013-01-01 21:00:00
#>  4 EWR    SJU          -6       -12 2013-01-01 21:02:00 2013-01-01 21:08:00
#>  5 EWR    SFO          11       -14 2013-01-01 21:08:00 2013-01-01 20:57:00
#>  6 LGA    FLL         -10        -2 2013-01-01 21:20:00 2013-01-01 21:30:00
#>  7 EWR    MCO          41        43 2013-01-01 21:21:00 2013-01-01 20:40:00
#>  8 JFK    LAX          -7       -24 2013-01-01 21:28:00 2013-01-01 21:35:00
#>  9 EWR    FLL          49        28 2013-01-01 21:34:00 2013-01-01 20:45:00
#> 10 EWR    FLL          -9       -14 2013-01-01 21:36:00 2013-01-01 21:45:00
#>    arr_time            sched_arr_time      air_time overnight
#>    <dttm>              <dttm>                 <dbl> <lgl>    
#>  1 2013-01-02 00:03:00 2013-01-03 00:07:00      192 TRUE     
#>  2 2013-01-02 00:29:00 2013-01-03 21:51:00       NA TRUE     
#>  3 2013-01-02 00:08:00 2013-01-03 23:59:00      159 TRUE     
#>  4 2013-01-02 01:46:00 2013-01-03 01:58:00      199 TRUE     
#>  5 2013-01-02 00:25:00 2013-01-03 00:39:00      354 TRUE     
#>  6 2013-01-02 00:16:00 2013-01-03 00:18:00      160 TRUE     
#>  7 2013-01-02 00:06:00 2013-01-03 23:23:00      143 TRUE     
#>  8 2013-01-02 00:26:00 2013-01-03 00:50:00      338 TRUE     
#>  9 2013-01-02 00:20:00 2013-01-03 23:52:00      152 TRUE     
#> 10 2013-01-02 00:25:00 2013-01-03 00:39:00      154 TRUE     
#> # ... with 10,623 more rows

flights_dt %>% 
  filter(overnight, arr_time < dep_time)
#> # A tibble: 0 x 10
#> # ... with 10 variables: origin <chr>, dest <chr>, dep_delay <dbl>,
#> #   arr_delay <dbl>, dep_time <dttm>, sched_dep_time <dttm>, arr_time <dttm>,
#> #   sched_arr_time <dttm>, air_time <dbl>, overnight <lgl>

Intervals

dyears(1) / ddays(365) returns one, because durations are always represented by a number of seconds, and a duration of a year is defined as 365 days worth of seconds.

For years(1) / days(1) it gives an estimate, and a warning.

dyears(1) / ddays(365)
#> [1] 1.000685

years(1) / days(1)
#> [1] 365.25

For more accurate measurements, use an interval. An interval is a duration with a starting point and hence you can determine how long it is.

(next_yr <- today() + years(1))
#> [1] "2021-11-10"
 
(today() %--% next_yr)
#> [1] 2020-11-10 UTC--2021-11-10 UTC

# to find how many periods in an _interval_ use integer division
(today() %--% next_yr) %/% days(1)
#> [1] 365

# for duration you can use / or integer division 
# for consistency probably %/% is better
(today() %--% next_yr) %/% ddays(1)
#> [1] 365

The allowed arithmetic operations between pairs of date/time classes.
The allowed arithmetic operations between pairs of date/time classes.

Exercises

  1. Why is there months() but no dmonths()?

    A month is not easily converted to a number of seconds, because a month could be 4 weeks, 5 weeks, 28 days, 30 days, or 31 days.

    For a day we use standard convention of 24 hours in a day, and convert that to seconds, same for a week (7 days of 24 each day). But with months there is no standard number.

  2. Explain days(overnight * 1) to someone who has just started learning R. How does it work?

    (flights_dt <- flights_dt %>% 
      mutate(overnight = arr_time < dep_time,
             arr_time = arr_time + days(overnight * 1),
             sched_arr_time = sched_arr_time + days(overnight + 1))) %>% 
      filter(overnight == TRUE) %>% 
      print(width = Inf)

    overnight is a logical value - in the df each arr_time is checked against dep_time if it is before the departure time?

    • it is: marked as TRUE (anomalies we are looking for).
    • it is not: marked as FALSE.

    We need to progress our anomaly arr_times on by 1 day.

    • For our anomalies overnight == TRUE which behind the scenes is the integer 1 (for yes, this criterion is met). \(overnight * 1 = 1 * 1 = 1\). The arrival time is progressed one day forward and is now accurate.
    • For normal flights (the arrival time is after the departure time) overnight == FALSE which behind the scenes is the integer 0 (for nope, this criterion is not met). \(overnight * 1 = 0 * 1 = 0\). The arrival time is unchanged since it is already correct.
  3. Create a vector of dates giving the first day of every month in 2015. Create a vector of dates giving the first day of every month in the current year.

    ymd(20150101) + months(0:11)
    #>  [1] "2015-01-01" "2015-02-01" "2015-03-01" "2015-04-01" "2015-05-01"
    #>  [6] "2015-06-01" "2015-07-01" "2015-08-01" "2015-09-01" "2015-10-01"
    #> [11] "2015-11-01" "2015-12-01"
    floor_date(today(), "year") + months(0:11)
    #>  [1] "2020-01-01" "2020-02-01" "2020-03-01" "2020-04-01" "2020-05-01"
    #>  [6] "2020-06-01" "2020-07-01" "2020-08-01" "2020-09-01" "2020-10-01"
    #> [11] "2020-11-01" "2020-12-01"
  4. Write a function that given your birthday (as a date), returns how old you are in years.

    age <- function(dob){
      secs <- as.duration(today() - dob)
      secs / (60 * 60 * 24 * 365)
    }
    
    age(ymd(20140612))
    #> [1] "6.41917808219178s"
  5. Why can’t (today() %--% (today() + years(1))) / months(1) work?

    It works but I suspect that the / should be replaced by %/% integer division.

    # how many months in the timespan today - next_yr_this_time
    (today() %--% (today() + years(1))) %/% months(1)
    #> [1] 12

Timezones

Timezones

# what is my timezone?
Sys.timezone()
#> [1] "Africa/Johannesburg"

Get a list of tz’s.

length(OlsonNames())
#> [1] 593
head(OlsonNames())
#> [1] "Africa/Abidjan"     "Africa/Accra"       "Africa/Addis_Ababa"
#> [4] "Africa/Algiers"     "Africa/Asmara"      "Africa/Asmera"
tail(OlsonNames())
#> [1] "US/Pacific-New" "US/Samoa"       "UTC"            "W-SU"          
#> [5] "WET"            "Zulu"

In Base R: the time zone is an attribute of the date-time that only controls printing.

(x1 <- ymd_hms("2015-06-01 12:00:00", tz = "America/New_York"))
#> [1] "2015-06-01 12:00:00 EDT"
(x2 <- ymd_hms("2015-06-01 18:00:00", tz = "Europe/Copenhagen"))
#> [1] "2015-06-01 18:00:00 CEST"
(x3 <- ymd_hms("2015-06-02 04:00:00", tz = "Pacific/Auckland"))
#> [1] "2015-06-02 04:00:00 NZST"

# verify same time
x1 - x2
#> Time difference of 0 secs
x1 - x3
#> Time difference of 0 secs

{lubridate} always uses UTC.

# tz dropped
x4 <- c(x1, x2, x3)
x4
#> [1] "2015-06-01 12:00:00 EDT" "2015-06-01 12:00:00 EDT"
#> [3] "2015-06-01 12:00:00 EDT"

# change tz for display
x4a <- with_tz(x4, tzone = "Australia/Lord_Howe")
x4a
#> [1] "2015-06-02 02:30:00 +1030" "2015-06-02 02:30:00 +1030"
#> [3] "2015-06-02 02:30:00 +1030"
x4a - x4
#> Time differences in secs
#> [1] 0 0 0

# change tz to fix an incorrect tz
x4b <- force_tz(x4, tzone = "Australia/Lord_Howe")
x4b
#> [1] "2015-06-01 12:00:00 +1030" "2015-06-01 12:00:00 +1030"
#> [3] "2015-06-01 12:00:00 +1030"
x4b - x4
#> Time differences in hours
#> [1] -14.5 -14.5 -14.5

sessionInfo()
#> R version 3.6.3 (2020-02-29)
#> Platform: x86_64-w64-mingw32/x64 (64-bit)
#> Running under: Windows 10 x64 (build 18363)
#> 
#> Matrix products: default
#> 
#> locale:
#> [1] LC_COLLATE=English_South Africa.1252  LC_CTYPE=English_South Africa.1252   
#> [3] LC_MONETARY=English_South Africa.1252 LC_NUMERIC=C                         
#> [5] LC_TIME=English_South Africa.1252    
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> other attached packages:
#>  [1] werpals_0.1.0              nycflights13_1.0.1        
#>  [3] tidyquant_1.0.0            quantmod_0.4.17           
#>  [5] TTR_0.23-6                 PerformanceAnalytics_2.0.4
#>  [7] xts_0.12-0                 zoo_1.8-7                 
#>  [9] lubridate_1.7.9            magrittr_1.5              
#> [11] flair_0.0.2                forcats_0.5.0             
#> [13] stringr_1.4.0              dplyr_1.0.2               
#> [15] purrr_0.3.4                readr_1.4.0               
#> [17] tidyr_1.1.2                tibble_3.0.3              
#> [19] ggplot2_3.3.2              tidyverse_1.3.0           
#> [21] workflowr_1.6.2           
#> 
#> loaded via a namespace (and not attached):
#>  [1] Rcpp_1.0.4.6     lattice_0.20-38  ps_1.3.2         utf8_1.1.4      
#>  [5] assertthat_0.2.1 rprojroot_1.3-2  digest_0.6.27    R6_2.4.1        
#>  [9] cellranger_1.1.0 backports_1.1.6  reprex_0.3.0     evaluate_0.14   
#> [13] httr_1.4.2       pillar_1.4.6     rlang_0.4.8      curl_4.3        
#> [17] readxl_1.3.1     rstudioapi_0.11  whisker_0.4      rmarkdown_2.4   
#> [21] labeling_0.3     munsell_0.5.0    broom_0.7.2      compiler_3.6.3  
#> [25] httpuv_1.5.2     modelr_0.1.8     xfun_0.13        pkgconfig_2.0.3 
#> [29] htmltools_0.5.0  tidyselect_1.1.0 emo_0.0.0.9000   quadprog_1.5-8  
#> [33] fansi_0.4.1      crayon_1.3.4     dbplyr_2.0.0     withr_2.2.0     
#> [37] later_1.0.0      Quandl_2.10.0    grid_3.6.3       jsonlite_1.7.1  
#> [41] gtable_0.3.0     lifecycle_0.2.0  DBI_1.1.0        git2r_0.26.1    
#> [45] scales_1.1.0     cli_2.1.0        stringi_1.5.3    farver_2.0.3    
#> [49] fs_1.5.0         promises_1.1.0   xml2_1.3.2       ellipsis_0.3.1  
#> [53] generics_0.0.2   vctrs_0.3.2      tools_3.6.3      glue_1.4.2      
#> [57] hms_0.5.3        yaml_2.2.1       colorspace_1.4-1 rvest_0.3.6     
#> [61] knitr_1.28       haven_2.3.1