A month can contain 28, 29, 30, or 31 days. A year can contain
365 or 366 days. Because of this inconsistency, the following operations can
give unexpected results:
- Adding or subtracting a number of months (or calculating a duration in
months) with a date that is on the 29th, 30th, or 31st of a month
- Adding or subtracting a number of years (or calculating a duration in
years) with a February 29 date.
The following rules are used: