Date-time classes (POSIXct and POSIXlt)

Introduction#

R includes two date-time classes — POSIXct and POSIXlt — see ?DateTimeClasses.

Remarks#

Pitfalls

With POSIXct, midnight will display only the date and time zone, though the full time is still stored.

Specialized packages

lubridate

Formatting and printing date-time objects

# test date-time object
options(digits.secs = 3)
d = as.POSIXct("2016-08-30 14:18:30.58", tz = "UTC")   

format(d,"%S")  # 00-61 Second as integer
## [1] "30"

format(d,"%OS") # 00-60.99… Second as fractional
## [1] "30.579"

format(d,"%M")  # 00-59 Minute
## [1] "18"

format(d,"%H")  # 00-23 Hours
## [1] "14"

format(d,"%I")  # 01-12 Hours
## [1] "02"

format(d,"%p")  # AM/PM Indicator
## [1] "PM"

format(d,"%z")  # Signed offset
## [1] "+0000"

format(d,"%Z")  # Time Zone Abbreviation
## [1] "UTC"

See ?strptime for details on the format strings here, as well as other formats.

Parsing strings into date-time objects

The functions for parsing a string into POSIXct and POSIXlt take similar parameters and return a similar-looking result, but there are differences in how that date-time is stored; see “Remarks.”

as.POSIXct("11:38",                        # time string
           format = "%H:%M")               # formatting string
## [1] "2016-07-21 11:38:00 CDT"           
strptime("11:38",                          # identical, but makes a POSIXlt object
         format = "%H:%M")
## [1] "2016-07-21 11:38:00 CDT"

as.POSIXct("11 AM",                   
           format = "%I %p")        
## [1] "2016-07-21 11:00:00 CDT"

Note that date and timezone are imputed.

as.POSIXct("11:38:22",                 # time string without timezone
           format = "%H:%M:%S",   
           tz = "America/New_York")    # set time zone
## [1] "2016-07-21 11:38:22 EDT"

as.POSIXct("2016-07-21 00:00:00",
           format = "%F %T")           # shortcut tokens for "%Y-%m-%d" and "%H:%M:%S"

See ?strptime for details on the format strings here.

Notes

Missing elements

If a date element is not supplied, then that from the current date is used.
If a time element is not supplied, then that from midnight is used, i.e. 0s.
If no timezone is supplied in either the string or the tz parameter, the local timezone is used.

Time zones

The accepted values of tz depend on the location.
- CST is given with "CST6CDT" or "America/Chicago"
For supported locations and time zones use:
- In R: OlsonNames()
- Alternatively, try in R: system("cat $R_HOME/share/zoneinfo/zone.tab")
These locations are given by Internet Assigned Numbers Authority (IANA)
- List of tz database time zones (Wikipedia)
- IANA TZ Data (2016e)

Date-time arithmetic

To add/subtract time, use POSIXct, since it stores times in seconds

## adding/subtracting times - 60 seconds
as.POSIXct("2016-01-01") + 60
# [1] "2016-01-01 00:01:00 AEDT"

## adding 3 hours, 14 minutes, 15 seconds
as.POSIXct("2016-01-01") + ( (3 * 60 * 60) + (14 * 60) + 15)
# [1] "2016-01-01 03:14:15 AEDT"

More formally, as.difftime can be used to specify time periods to add to a date or datetime object. E.g.:

as.POSIXct("2016-01-01")         + 
  as.difftime(3,  units="hours") +
  as.difftime(14, units="mins")  +
  as.difftime(15, units="secs")
# [1] "2016-01-01 03:14:15 AEDT"

To find the difference between dates/times use difftime() for differences in seconds, minutes, hours, days or weeks.

# using POSIXct objects
difftime(
  as.POSIXct("2016-01-01 12:00:00"), 
  as.POSIXct("2016-01-01 11:59:59"), 
  unit = "secs")
# Time difference of 1 secs

To generate sequences of date-times use seq.POSIXt() or simply seq.