Month Calculator
Enter a start and end date to count the exact number of months (and remaining days) between them. Switch to "Add / subtract months" to find the date after adding or removing a number of months, or use "Months since" to count months elapsed from any past month and year. All three modes update instantly as you type.
How this calculator counts months
The calculator uses calendar-month arithmetic rather than a fixed 30-day approximation. To find the number of full months between two dates, it converts each date into a total-month index (year x 12 + month), subtracts start from end, then adjusts by one month if the end day falls before the start day. The leftover days are the days remaining in the partial month. This mirrors how mortgage lenders, subscription services, and HR systems measure month-to-month periods, and it respects February, 30-day months, and leap years automatically.
Add or subtract months from a date
The "Add / subtract months" mode adds (or removes) a whole number of months to any date. Behind the scenes it adds the requested count to the month index and wraps the year accordingly. If the source day cannot exist in the target month (for example, March 31 minus one month gives February, which has at most 29 days), the day is clamped to the last valid day of the target month. Enter a negative number in the months field to go backward in time.
Months since a month and year
The "Months since" mode counts how many full months have elapsed from the first day of a past month up to today. It is useful for calculating the age of a subscription, project, or relationship measured in months rather than years. The partial month is shown separately as remaining days so you can see exactly where you are within the current month.
Why months are not all the same length
Calendar months range from 28 to 31 days. February has 28 days in a common year and 29 in a leap year (years divisible by 4, except century years not divisible by 400). This means a "month" is not a fixed number of days, which is why period calculators that simply multiply by 30 or 30.44 give slightly different answers than calendar-accurate tools. This calculator counts months by moving along the calendar rather than by dividing a day count, so the result matches what a calendar on a wall would show.
Common time-span reference
| Period | Months | Approximate days |
|---|---|---|
| 1 quarter | 3 | 91 |
| Half year (6 months) | 6 | 182 |
| 1 year | 12 | 365 |
| 18 months | 18 | 548 |
| 2 years | 24 | 730 |
| 3 years | 36 | 1095 |
| 4 years | 48 | 1461 |
| 5 years | 60 | 1826 |
| 10 years | 120 | 3652 |
Handy reference for frequently used durations expressed in months.
Frequently asked questions
How many months are in a year?
There are exactly 12 months in a Gregorian calendar year. A calendar year spans 365 days in a common year and 366 days in a leap year, giving an average of approximately 30.44 days per month.
How do I calculate months between two dates?
Subtract the start year from the end year and multiply by 12, then add the difference in month numbers. If the end day is earlier than the start day, subtract 1 from the result and count the leftover days separately. Example: from March 15 to November 5 is (11 - 3) = 8 months minus 1 (because 5 < 15) = 7 full months and 21 leftover days.
What happens when I add months to a date that ends on the 31st?
If the target month has fewer days than the source day, the result is clamped to the last day of the target month. For example, adding 1 month to January 31 gives February 28 (or 29 in a leap year), not March 3.
Is 6 months exactly 180 days?
Not always. Six calendar months can span 181, 182, or 183 days depending on which months are included and whether a leap year February falls in the range. The exact day count varies, which is why this calculator shows both full months and total days.
How do I find what date is 3 months from today?
Use the "Add / subtract months" mode, set the starting date to today, and enter 3 in the months field. The calculator shows the resulting date. If today is the 31st and the target month has fewer than 31 days, the day is clamped to the last valid day.
Can I count months between past and future dates?
Yes. Enter any start and end date in "Months between two dates" mode, including future dates. If the end date is earlier than the start date, the result is shown as a negative number of months.