Add days to date
Thirty-one January plus one month is 28 February, not 3 March — the day clamps to the end of a shorter month. Plus thirty days would give 2 March instead. Contracts almost always mean calendar months, which is why the unit selector matters more than it looks.
Adding days is simple arithmetic; adding months is not. Thirty-one January plus one month gives 28 February, clamping to the end of the shorter month, whereas plus thirty days gives 2 March. Both are correct answers to different questions.
How to add to a date
Clamping is the convention almost every system uses and it is worth knowing it is a convention rather than a fact. Thirty-one January plus one month could defensibly be 3 March, on the grounds that it preserves the day count. Every major date library and every legal reading picks the end of February instead, because a monthly obligation falling due "on the 31st" should not skip a month entirely. This tool follows that.
Questions
Set the amount to 30 and read the result — the weekday is shown too.