Example: Date/Time

Coercing data types to date and/or time information is generally performed using as.Date or either as.POSIXct or as.POSIXlt. Let’s start with as.Date:

as.Date("2014-01-01 13:30:35")

## [1] "2014-01-01"

If the character string supplied to the function comes in the ISO standard format, as.Date will notice it automatically. Otherwise, we have to define the format:

as.Date("01.01.2014", "%m.%d.%Y")

## [1] "2014-01-01"

While as.Date requires information on the year, month and day withouth any exception, time information is not handled at all.

If data and time or just time information is required to be handeled explicitly, the temporal information can be converted to a POSIX data type. In general, there are two kinds of POSIX classes: (i) POSIXct which handles date/time information as seconds since a certain time (standard: 1970-01-01 00:00.00 UTC) and (ii) POSIXlt which stores date/time information as a list. Again, if the data/time information is not supplied in standard format, one has to define it:

as.POSIXct("01.01.2014 13:30:35", format = "%m.%d.%Y %H:%M:%S")

## [1] "2014-01-01 13:30:35 CET"

as.POSIXlt("01.01.2014 13:30:35", format = "%m.%d.%Y %H:%M:%S")

## [1] "2014-01-01 13:30:35 CET"

For an overview of the formatting syntax, see the help pages of the strptime function, which handles the conversion. The latter function can also be used directly for converting character information to POSIXlt:

lt <- strptime("01.01.2014 13:30:35", "%m.%d.%Y %H:%M:%S")
lt

## [1] "2014-01-01 13:30:35 CET"

class(lt)

## [1] "POSIXlt" "POSIXt"

This function is faster than as.POSIXlt if the time format is supplied in the ISO way. Otherwise (and maybe even in this case), just forget about it.

Aside from theory, a pratical difference between the POSIXct and POSIXlt class is the way one can add or substract a specific amount of time. For POSIXct, you have to supply the time interval as seconds:

ct <- as.POSIXct("01.01.2014 13:30:35", format = "%m.%d.%Y %H:%M:%S")
ct + 60 * 30  # add 30 minutes to ct

## [1] "2014-01-01 14:00:35 CET"

While this can also be used for POSIXlt, one can also access the individual time information directly:

lt <- as.POSIXlt("01.01.2014 13:30:35", format = "%m.%d.%Y %H:%M:%S")
lt$hour <- lt$hour + 24  # add 24 hours
lt

## [1] "2014-01-02 13:30:35 CET"

To convert a POSIX information back to a character string, one can use the strftime function which allows precise formatting of the data/time string and also has an option for time zone conversion.

ct

## [1] "2014-01-01 13:30:35 CET"

strftime(ct, "%d. %b %y, %H:%M", tz = "UTC")

## [1] "01. Jan 14, 12:30"

Please note that the time zone conversion only works for POSIXct.

Finally, if you want to substract two date/time values, you can use the arithmetic operation or the difftime function to control the layout of the result (see the help pages of strptime for an overview of the layout specifications).

lt

## [1] "2014-01-02 13:30:35 CET"

ct

## [1] "2014-01-01 13:30:35 CET"

lt - ct

## Time difference of 1 days

difftime(lt, ct, units = "mins")

## Time difference of 1440 mins