Add or subtract from a date

How months and years are added

Years and months are applied first, then weeks and days. Adding a month keeps the same day number when the target month has it. When it does not, the result is the last day of that month, so January 31 plus one month is February 28 in a common year and February 29 in a leap year, and March 31 minus one month is the last day of February.

A year is treated as twelve months, so February 29 plus one year is February 28 of the next year, and plus four years is February 29 again.

Weeks are exactly seven days. Days are calendar days, so the result is never thrown off by daylight saving time.