Counting months is harder than it looks because months are not a fixed length. This calculator counts by calendar anniversary: it finds the largest number of whole months that fits between the two dates, then reports the days left over. That is the convention used for rent, subscriptions and loan terms.
End-of-month dates need a rule, and the rule here is clamping. From 31 January, one month later is 28 or 29 February rather than spilling into March. Without clamping, adding one month twice from 31 January would land you on 3 March, which no lease or billing system would accept.
From 30 November 2025 to 9 August 2026 is 8 whole months plus 10 days. The decimal figure divides the leftover days by the length of the month they fall in, so it never assumes 30.
The formula
A- Start date
B- End date
M- Whole calendar months elapsed
R- Days after the last whole month
L- Length of the month the remainder falls in, used for the decimal
How it works, step by step
- Enter the start date.
- Enter the end date.
- The gauge shows months as a decimal.
- The breakdown gives whole months, leftover days, quarters and the average days per month.
Worked examples
30 November 2025 to 9 August 2026
That is 8 whole months and 10 days. Starting on the 30th matters: from 30 November, one month later is 30 December 2025, and the day-of-month is preserved wherever the target month is long enough.
The clamping rule in action
From 31 January 2026, adding one month gives 28 February 2026, and adding two gives 31 March 2026. In 2024, a leap year, one month from 31 January gives 29 February 2024 instead.
How to read your score
Frequently asked questions
Why not just divide the days by 30.44?
That gives an average, not a calendar answer. It will tell you 31 January to 28 February is 0.92 months when every lease, bank and court would call it exactly one month. Averages are useful for statistics, not for counting anniversaries.
How is the decimal part calculated?
By dividing the leftover days by the actual length of the month they sit in. If the remainder falls in a 31-day month, 15 days is 0.48 of a month; in February it would be 0.54.
What happens with end-of-month start dates?
The day of month is clamped to the last day of the target month, so 31 August plus one month is 30 September. The original day is remembered, so adding two months returns you to 31 October.
Does this match how my lender counts months?
Almost certainly, since calendar-anniversary counting with clamping is the near-universal convention for loans, leases and subscriptions. Check the contract if a single day matters financially.
Month length reference
| Month | Common year | Leap year |
|---|---|---|
| January | 31 days | 31 days |
| February | 28 days | 29 days |
| March | 31 days | 31 days |
| April | 30 days | 30 days |
| May | 31 days | 31 days |
| June | 30 days | 30 days |
| July | 31 days | 31 days |
| August | 31 days | 31 days |
| September | 30 days | 30 days |
| October | 31 days | 31 days |
| November | 30 days | 30 days |
| December | 31 days | 31 days |
Total 365 days in a common year and 366 in a leap year. The average Gregorian month is 30.4369 days.