Add Months to a Date takes a starting date and a number of months and works out where that lands — straightforward for most dates, but genuinely tricky at the edges, which is exactly where this calculator earns its keep. Unlike weeks, months don't have a fixed length, so "add one month" has to make a judgement call whenever the starting day doesn't exist in the target month.
The standard rule, and the one used here, is that adding a month to 31 January gives you the last day of February — the 28th, or the 29th in a leap year — rather than an invalid "31 February." The same logic applies any time a start date falls on the 29th, 30th or 31st and the target month is shorter: the result rolls back to that month's actual last day instead of overflowing into the next one.
This matters for anything billed or scheduled monthly — subscriptions, rent, loan repayments, contract renewals — where an end-of-month start date needs a sensible answer every time, not just for months with 31 days. If your starting date isn't near the end of a month, adding months behaves exactly as expected, landing on the same day number in the target month.
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