[jimminy]

If you're adding 1 month, you're working on the month's location, e.g. 2021 - (02) - 28.

When you increment the month, the result would be 2021-03-28.

The only time you'd modify the day, is if the day became invalid due to an overflow, during that increment. If, when, you overflow the days you'd set the value of days to the maximum in that month.

If I tell someone, I'll get to that in a month, they expect by this day in the next month, the next calendar page, not 30/31 days.