Month-End Overflow: Why January 31 Plus One Month Is Tricky
Adding months is not like adding days. Here is the clamping rule, and why it is the honest choice.
Months are uneven
Days are uniform, but months are not: they run from 28 to 31 days. So adding one month to 31 January cannot land on 31 February, because that date does not exist. Something has to give, and every calculator must choose a rule.
The clamping rule
This site clamps to the last day of the target month: 31 January plus one month becomes 28 February (29 in a leap year), and 31 March plus one month becomes 30 April. The alternative — rolling over into the next month, so 31 January plus one month becomes 3 March — looks clever but surprises most people, because the result is no longer in the month they asked for.
When it matters
Month-end arithmetic matters for subscriptions, rent, loan dates and contract deadlines. If a contract says monthly from 31 January, check what it actually says about short months before assuming a date. For planning, the clamping rule is the one most people expect.
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