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R - Date-time classes (POSIXct and POSIXlt)
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  Formatting and printing date-time objects

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

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