Adding months frustrates basic arithmetic because consecutive months have
different lengths. With other elements, it is helpful for arithmetic to
perform automatic roll over. For example, 12:00:00 + 61 seconds becomes
12:01:01. However, people often prefer that this behavior NOT occur with
months. For example, we sometimes want January 31 + 1 month = February 28 and
not March 3.
%m+% performs this type of arithmetic. Date
always returns a date in the nth month after Date. If the new date would
usually spill over into the n + 1th month,
%m+% will return the last day of
the nth month (
%m-% months(n) always returns a
date in the nth month before Date.
add_with_rollback() provides additional functionality to
%m-%. It allows rollback to first day of the month instead of the last day
of the previous month and controls whether HMS component of the end date is
preserved or not.
e1 %m+% e2 add_with_rollback(e1, e2, roll_to_first = FALSE, preserve_hms = TRUE)
rollback to the first day of the month instead of the
last day of the previous month (passed to
retains the same hour, minute, and second information? If
FALSE, the new date will be at 00:00:00 (passed to
A date-time object of class POSIXlt, POSIXct or Date
%m-% handle periods with components less than a month by first
adding/substracting months and then performing usual arithmetics with smaller
%m-% should be used with caution as they are not one-to-one
operations and results for either will be sensitive to the order of
jan <- ymd_hms("2010-01-31 03:04:05") jan + months(1:3) # Feb 31 and April 31 returned as NA#>  NA "2010-03-31 03:04:05 UTC" #>  NA# NA "2010-03-31 03:04:05 UTC" NA jan %m+% months(1:3) # No rollover#>  "2010-02-28 03:04:05 UTC" "2010-03-31 03:04:05 UTC" #>  "2010-04-30 03:04:05 UTC"leap <- ymd("2012-02-29") "2012-02-29 UTC"#>  "2012-02-29 UTC"leap %m+% years(1)#>  "2013-02-28"leap %m+% years(-1)#>  "2011-02-28"leap %m-% years(1)#>  "2011-02-28"