A month is a position on the calendar
January to February is a calendar-month move even though February is shorter. Counting a fixed number of days is a different operation. Choose the unit that describes your intention before you make the calculation.
Decide what happens at the end of a month
A January 31 anniversary has no exact February 31 equivalent. AgeUse uses the last day of the destination month in this situation. It always measures complete months from the original starting date, rather than repeatedly adding one shortened month.
Keep the anchor date visible
For January 31 to March 31, the completed-month count is two. For January 31 to March 30, the March anniversary has not arrived; the result uses the completed February anniversary plus the remaining days. The original date is the anchor for both calculations.