Add and subtract months to a date without exceeding the last day of the new month

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 %m+% months(n) 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 (rollback()). Date %m-% months(n) always returns a date in the nth month before Date.

e1 %m+% e2

add_with_rollback(e1, e2, roll_to_first = FALSE, preserve_hms = TRUE)

Arguments

e1	A period or a date-time object of class POSIXlt, POSIXct or Date.
e2	A period or a date-time object of class POSIXlt, POSIXct or Date. Note that one of e1 and e2 must be a period and the other a date-time object.
roll_to_first	rollback to the first day of the month instead of the last day of the previous month (passed to rollback())
preserve_hms	retains the same hour, minute, and second information? If FALSE, the new date will be at 00:00:00 (passed to rollback())

Value

A date-time object of class POSIXlt, POSIXct or Date

Details

%m+% and %m-% handle periods with components less than a month by first adding/subtracting months and then performing usual arithmetics with smaller units.

%m+% and %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 operations.

Examples

jan <- ymd_hms("2010-01-31 03:04:05")
jan + months(1:3) # Feb 31 and April 31 returned as NA
#> [1] NA                        "2010-03-31 03:04:05 UTC"
#> [3] NA                       
# NA "2010-03-31 03:04:05 UTC" NA
jan %m+% months(1:3) # No rollover
#> [1] "2010-02-28 03:04:05 UTC" "2010-03-31 03:04:05 UTC"
#> [3] "2010-04-30 03:04:05 UTC"

leap <- ymd("2012-02-29")
"2012-02-29 UTC"
#> [1] "2012-02-29 UTC"
leap %m+% years(1)
#> [1] "2013-02-28"
leap %m+% years(-1)
#> [1] "2011-02-28"
leap %m-% years(1)
#> [1] "2011-02-28"