Calendar arithmetic is harder than it looks: months have 28–31 days, leap years bend February, and adding ‘1 month’ to Jan 31 has no single right answer.

We follow the common convention: add months first (clamping to the last valid day), then days. So Jan 31 + 1 month = Feb 28 (or 29 in a leap year).

The Difference mode uses the same rule: we count whole calendar months, then leftover days — the same way human beings phrase ages.

Use total days/weeks/hours for contracts, billing cycles or project deadlines where precision matters more than calendar phrasing.