Month Calculator

Months are the most awkward unit in the calendar. They range from 28 to 31 days, which means “three months from now” is not a fixed number of days and never has been.

What you can work out

1. Enter two dates to count the whole months between them.
2. Or enter a date and a number of months to find the resulting date.
3. Use negative numbers to count backwards.
4. Check the leftover days, since months rarely divide evenly.
5. Note how the end-of-month cases are handled before relying on the result.

The end-of-month problem

What is one month after the 31st of January? There is no 31st of February, so something has to give. The near-universal convention is to clamp to the last valid day of the target month, giving the 28th of February, or the 29th in a leap year.

This has a quirk worth knowing: the operation is not reversible. One month forward from 31 January lands on 28 February, and one month back from 28 February lands on 28 January, not the 31st. Contracts and subscription billing systems deal with this explicitly, and it is worth checking which convention yours uses.

Where month counts are used

• Subscription and billing cycles.
• Loan and mortgage terms, which are almost always expressed in months.
• Employment probation periods and notice requirements.
• Warranty and guarantee lengths.
• Age in months for infants, which is how paediatric milestones are tracked.

Months versus weeks

A month is not four weeks. Four weeks is 28 days; an average month is about 30.44. Over a year that difference adds up to a full extra month — which is exactly why there are 13 four-week pay periods in a year but only 12 monthly ones. If you are budgeting, pick one convention and stay with it.

Month calculation questions