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There is no fixed number of days in a MONTH interval. DATE_SUB is mostly just decrementing the number in the months position of the date provided. So DATE_SUB('2020-09-14', INTERVAL 3 MONTH) is '20...
Answer
#1: Initial revision
by
r~~
·
2020-09-14T18:35:17Z (about 4 years ago)
There is no fixed number of days in a `MONTH` interval. `DATE_SUB` is mostly just decrementing the number in the months position of the date provided. So `DATE_SUB('2020-09-14', INTERVAL 3 MONTH)` is `'2020-06-14'` and `DATE_SUB('2020-03-14', INTERVAL 3 MONTH)` is `'2019-12-14'`. [The documentation](https://dev.mysql.com/doc/refman/8.0/en/expressions.html#temporal-intervals) describes what happens in edge cases like `DATE_SUB('2020-10-31', INTERVAL 1 MONTH)`:
> If you add `MONTH`, `YEAR_MONTH`, or `YEAR` and the resulting date has a day that is larger than the maximum day for the new month, the day is adjusted to the maximum days in the new month
So the result of `DATE_SUB('2020-10-31', INTERVAL 1 MONTH)` would be adjusted to `'2020-09-30'`.