-Dealing with Dates

#Getting current date & time

Sys.timezone()
[1] "Europe/Berlin"
> Sys.Date()
[1] "2018-09-07"
> Sys.time()
[1] "2018-09-07 12:58:09 CEST"

If using the lubridate package:

library(lubridate)
now()

#Converting strings to dates

Convert Strings to Dates:

x <- c("2015-07-01", "2015-08-01", "2015-09-01")

as.Date(x)
## [1] "2015-07-01" "2015-08-01" "2015-09-01"

Note that the default date format is YYYY-MM-DD

use?strftimein your console to know the date format

y <- c("07/01/2015", "07/01/2015", "07/01/2015")

as.Date(y, format = "%m/%d/%Y")
## [1] "2015-07-01" "2015-07-01" "2015-07-01"

Using the lubridate package:

library(lubridate)
ymd(x)
## [1] "2015-07-01 UTC" "2015-08-01 UTC" "2015-09-01 UTC"

mdy(y)
## [1] "2015-07-01 UTC" "2015-07-01 UTC" "2015-07-01 UTC"

lubridate package recogniced the common separators used when recording dates (“-“, “/”, “.”, and “”).

Create Dates by Merging Data:

yr <- c("2012", "2013", "2014", "2015")
mo <- c("1", "5", "7", "2")
day <- c("02", "22", "15", "28")

# ISOdate converts to a POSIXct object
ISOdate(year = yr, month = mo, day = day)
## [1] "2012-01-02 12:00:00 GMT" "2013-05-22 12:00:00 GMT"
## [3] "2014-07-15 12:00:00 GMT" "2015-02-28 12:00:00 GMT"

# truncate the unused time data by converting with as.Date
as.Date(ISOdate(year = yr, month = mo, day = day))

#Extract & manipulate parts of dates

Manipulating data with lubridate

library(lubridate)
x <- c("2015-07-01", "2015-08-01", "2015-09-01")

year(x)
## [1] 2015 2015 2015

# default is numerical value
month(x)
## [1] 7 8 9

# show abbreviated name
month(x, label = TRUE)
## [1] Jul Aug Sep
## 12 Levels: 
#Jan < Feb < Mar < Apr < May < Jun < Jul < Aug < Sep < ... < Dec

# show unabbreviated name
month(x, label = TRUE, abbr = FALSE)
## [1] July      August    September
## 12 Levels: 
#January < February < March < April < May < June < ... < December


wday(x, label = TRUE, abbr = FALSE)
## [1] Wednesday Saturday  Tuesday  
## 7 Levels: 
#Sunday < Monday < Tuesday < Wednesday < Thursday < ... < Saturday

Change the values of date elements

# convert to date format
x <- ymd(x)
x
## [1] "2015-07-01 UTC" "2015-08-01 UTC" "2015-09-01 UTC"

# change the days for the dates
mday(x)
## [1] 1 1 1

mday(x) <- c(3, 10, 22)
x
## [1] "2015-07-03 UTC" "2015-08-10 UTC" "2015-09-22 UTC"

# can also use 'update()' function
update(x, year = c(2013, 2014, 2015), month = 9)
## [1] "2013-09-03 UTC" "2014-09-10 UTC" "2015-09-22 UTC"

# can also add/subtract units
x + years(1) - days(c(2, 9, 21))
## [1] "2016-07-01 UTC" "2016-08-01 UTC" "2016-09-01 UTC"

#Creating date sequences

seq(as.Date("2010-1-1"), as.Date("2015-1-1"), by = "years")
## [1] "2010-01-01" "2011-01-01" "2012-01-01" "2013-01-01" "2014-01-01"
## [6] "2015-01-01"

seq(as.Date("2015/1/1"), as.Date("2015/12/30"), by = "quarter")
## [1] "2015-01-01" "2015-04-01" "2015-07-01" "2015-10-01"

seq(as.Date('2015-09-15'), as.Date('2015-09-30'), by = "2 days")
## [1] "2015-09-15" "2015-09-17" "2015-09-19" "2015-09-21" "2015-09-23"
## [6] "2015-09-25" "2015-09-27" "2015-09-29"

Using the lubridate package.

library(lubridate)
seq(ymd("2010-1-1"), ymd("2015-1-1"), by = "years")
## [1] "2010-01-01 UTC" "2011-01-01 UTC" "2012-01-01 UTC" "2013-01-01 UTC"
## [5] "2014-01-01 UTC" "2015-01-01 UTC"

seq(ymd("2015/1/1"), ymd("2015/12/30"), by = "quarter")
## [1] "2015-01-01 UTC" "2015-04-01 UTC" "2015-07-01 UTC" "2015-10-01 UTC"

seq(ymd('2015-09-15'), ymd('2015-09-30'), by = "2 days")
## [1] "2015-09-15 UTC" "2015-09-17 UTC" "2015-09-19 UTC" "2015-09-21 UTC"
## [5] "2015-09-23 UTC" "2015-09-25 UTC" "2015-09-27 UTC" "2015-09-29 UTC"

Creating sequences with time is very similar; however, we need to make sure our date object is POSIXct rather than just a Date object (as produced by as.Date):

seq(as.POSIXct("2015-1-1 0:00"), as.POSIXct("2015-1-1 12:00",by = "hour")
##  [1] "2015-01-01 00:00:00 EST" "2015-01-01 01:00:00 EST"
##  [3] "2015-01-01 02:00:00 EST" "2015-01-01 03:00:00 EST"
##  [5] "2015-01-01 04:00:00 EST" "2015-01-01 05:00:00 EST"
##  [7] "2015-01-01 06:00:00 EST" "2015-01-01 07:00:00 EST"
##  [9] "2015-01-01 08:00:00 EST" "2015-01-01 09:00:00 EST"
## [11] "2015-01-01 10:00:00 EST" "2015-01-01 11:00:00 EST"
## [13] "2015-01-01 12:00:00 EST"

# with lubridate
seq(ymd_hm("2015-1-1 0:00"), ymd_hm("2015-1-1 12:00"), by = "hour")
##  [1] "2015-01-01 00:00:00 UTC" "2015-01-01 01:00:00 UTC"
##  [3] "2015-01-01 02:00:00 UTC" "2015-01-01 03:00:00 UTC"
##  [5] "2015-01-01 04:00:00 UTC" "2015-01-01 05:00:00 UTC"
##  [7] "2015-01-01 06:00:00 UTC" "2015-01-01 07:00:00 UTC"
##  [9] "2015-01-01 08:00:00 UTC" "2015-01-01 09:00:00 UTC"
## [11] "2015-01-01 10:00:00 UTC" "2015-01-01 11:00:00 UTC"
## [13] "2015-01-01 12:00:00 UTC"

#Calculations with dates

R stores date and time objects as numbers. But we can calculated it such as logical comparisons, addition, subtraction, and working with durations.

x <- Sys.Date()
x
## [1] "2015-09-26"

y <- as.Date("2015-09-11")

x > y
## [1] TRUE

x - y
## Time difference of 15 days

# last leap year
x <- as.Date("2012-03-1")
y <- as.Date("2012-02-28")

x - y
## Time difference of 2 days


# example with time zones
x <- as.POSIXct("2015-09-22 01:00:00", tz = "US/Eastern")
y <- as.POSIXct("2015-09-22 01:00:00", tz = "US/Pacific")

y == x
## [1] FALSE

y - x
## Time difference of 3 hours

With the lubridate package

library(lubridate)

x <- now()
x
## [1] "2015-09-26 10:08:18 EDT"

y <- ymd("2015-09-11")

x > y
## [1] TRUE

x - y
## Time difference of 15.5891 days

y + days(4)
## [1] "2015-09-15 UTC"

x - hours(4)
## [1] "2015-09-26 06:08:18 EDT"

# create new duration (represented in seconds)
new_duration(60)
## [1] "60s"

# create durations for minutes, hours, years
dminutes(1)
## [1] "60s"

dhours(1)
## [1] "3600s (~1 hours)"

dyears(1)
## [1] "31536000s (~365 days)"

# add/subtract durations from date/time object
x <- ymd_hms("2015-09-22 12:00:00")

x + dhours(10)
## [1] "2015-09-22 22:00:00 UTC"

x + dhours(10) + dminutes(33) + dseconds(54)
## [1] "2015-09-22 22:33:54 UTC"

#Dealing with time zones & daylight savings

Change the time zone for a date/time with the with_tz() function.

library(lubridate)

time <- now()
time
## [1] "2015-09-26 10:30:32 EDT"

with_tz(time, tzone = "MST")
## [1] "2015-09-26 07:30:32 MST"

Change the time zone without changing the clock time with force_tz().

time
## [1] "2015-09-26 10:30:32 EDT"

force_tz(time, tzone = "MST")
## [1] "2015-09-26 10:30:32 MST"

Savings times to eliminate impacts on date/time calculations.

# most recent daylight savings time
ds <- ymd_hms("2015-03-08 01:59:59", tz = "US/Eastern")

# if we add a duration of 1 sec we gain an extra hour
ds + dseconds(1)
## [1] "2015-03-08 03:00:00 EDT"

# add a duration of 2 hours will reflect actual daylight 
#savings clock time 
# that occured 2 hours after 01:59:59 on 2015-03-08
ds + dhours(2)
## [1] "2015-03-08 04:59:59 EDT"

# add a period of two hours will reflect clock time that normally 
#occurs after
# 01:59:59 and is not influenced by daylight savings time.
ds + hours(2)
## [1] "2015-03-08 03:59:59 EDT"

#Additional resources

• Dates and times made easy with lubridate

• Date and time classes in R