Timespan-class
class. Periods track the
change in the "clock time" between two date-times. They
are measured in common time related units: years, months,
days, hours, minutes, and seconds. Each unit except for
seconds must be expressed in integer values.Interval-class
object with
as.interval
or by adding it to a date-time
with "+". Periods provide a method for measuring generalized
timespans when we wish to model clock times. Periods will
attain intuitive results at this task even when leap
years, leap seconds, gregorian days, daylight savings
changes, and other events happen during the period. See
Duration-class
for an alternative way to
measure timespans that allows precise comparisons between
timespans.
The logic that guides arithmetic with periods can be
unintuitive. Starting with version 1.3.0, lubridate
enforces the reversible property of arithmetic (e.g. a
date + period - period = date) by returning an NA if you
create an implausible date by adding periods with months
or years units to a date. For example, adding one month
to January 31st, 2013 results in February 31st, 2013,
which is not a real date. lubridate users have argued in
the past that February 31st, 2013 should be rolled over
to March 3rd, 2013 or rolled back to February 28, 2013.
However, each of these corrections would destroy the
reversibility of addition (Mar 3 - one month == Feb 3 !=
Jan 31, Feb 28 - one month == Jan 28 != Jan 31). If you
would like to add and subtract months in a way that rolls
the results back to the last day of a month (when
appropriate) use the special operators,
%m+%
and %m-%
.
Period class objects have six slots. 1) .Data, a numeric object. The apparent amount of seconds to add to the period. 2) minute, a numeric object. The apparent amount of minutes to add to the period. 3) hour, a numeric object. The apparent amount of hours to add to the period.4) day, a numeric object. The apparent amount of days to add to the period.5) month, a numeric object. The apparent amount of months to add to the period. 6) year, a numeric object. The apparent amount of years to add to the period.