Month Calculator
Add or subtract months from a date, with month-end handled correctly.
How to use the Month Calculator
- Pick a start date.
- Enter the number of months and choose add or subtract.
- Read the result date — 31 Jan + 1 month becomes 28/29 Feb, not 3 March.
Result = start date ± months (clamped to a valid day)
About the Month Calculator
This calculator adds or subtracts whole months from a date and returns the resulting date, using result = start date +/- months with month-end handled correctly. Its distinguishing feature is the clamp: when a start day does not exist in the target month, it lands on the last valid day of that month rather than overflowing into the next one.
The classic example is 31 January plus one month. There is no 31 February, so the tool returns 28 February (or 29 in a leap year), not 3 March. This matches how contracts, subscriptions, and billing cycles are normally understood, where adding a month means the same-numbered day where possible and the month-end otherwise. Naive date maths that simply overflows the extra days gives the wrong answer here.
Use it for anything measured in months: a six-month renewal, a three-month probation period, a loan or lease term, or a subscription end date. Pick the start date, enter the number of months, and choose add or subtract. To shift a date by individual days instead, use the Days Calculator, or measure the span between two dates with the Date Difference Calculator.
Frequently asked questions
What does adding a month mean in this tool?
It moves to the same day number in the target month where that day exists. When it does not, such as the 31st in a 30-day month, the result clamps to the last day of that month rather than spilling into the next.
What does 31 January plus one month give?
It gives 28 February, or 29 February in a leap year, because February has no 31st. The tool clamps to the month-end instead of rolling over to early March, which matches how billing and contract dates usually work.
Why not just add 30 or 31 days?
Because months vary in length, adding a fixed number of days drifts away from the same calendar day. Adding months keeps you on the corresponding date, which is how renewals, terms and subscriptions are normally counted.
Can I subtract months as well?
Yes. Choose the subtract option to move backward by the number of months you enter. The same month-end clamping applies, so the result always lands on a valid calendar date.
Does the clamp affect leap years?
Yes. When the target is February, the clamp uses 29 in a leap year and 28 otherwise, so adding a month to a 29th, 30th or 31st start date always resolves to the correct final day of that February.