Months Calculator — Calculate Months Between Dates
Calculate the number of months between two dates or add/subtract months from a date. See also Weeks Calculator and Years Calculator.
How the Months Calculator Works
The Months Calculator computes the exact number of calendar months between two dates, or determines a new date by adding or subtracting months from a starting date. Unlike simple 30-day approximations, this calculator uses actual calendar months which vary from 28 to 31 days. It properly handles month-end edge cases — for example, adding 1 month to January 31 results in February 28 (or 29 in a leap year), since February 31 does not exist.
Month Calculation Formula
Months Between = (End Year - Start Year) x 12 + (End Month - Start Month)
If End Day < Start Day: subtract 1 month, add previous month's days to remaining
Add Months: New Date = Start Date with month incremented by N
If resulting day overflows month: clamp to last day of target month
Month-End Handling Example
January 31 + 1 month = February 28 (or 29 in leap year)
March 31 + 1 month = April 30 (April has only 30 days)
August 31 - 1 month = July 31 (July has 31 days, no adjustment)
Days in Each Month Reference
| Month | Days | Month | Days |
|---|---|---|---|
| January | 31 | July | 31 |
| February | 28/29 | August | 31 |
| March | 31 | September | 30 |
| April | 30 | October | 31 |
| May | 31 | November | 30 |
| June | 30 | December | 31 |
Frequently Asked Questions
How are months calculated when days don't match?
When the start day is greater than the end day, we borrow from the previous month. For example, from January 31 to March 5: January 31 to February 28 is not a full month (since 31 > 28), so we count 0 months and calculate the remaining days from January 31 to March 5 = 1 month and 5 days.
What happens when adding months to the 31st?
If the target month has fewer days than the source day, the date is clamped to the last day of the target month. For example, January 31 + 1 month = February 28 (or 29 in a leap year). This is the standard behavior used by most programming languages and calendar applications.
How many months are in a year?
There are exactly 12 months in a Gregorian calendar year. The total number of days varies: 365 in a regular year and 366 in a leap year. The average month length is approximately 30.44 days (365.25 / 12).
Is there a difference between calendar months and 30-day months?
Yes. A calendar month is the period from a date in one month to the same date in the next month (e.g., March 15 to April 15). A "30-day month" is a fixed 30-day period. This calculator uses calendar months for accuracy, which is how most legal, financial, and business contexts define months.
How do I calculate months for a lease or contract?
Enter the contract start date and end date to find the exact number of months. Most leases use calendar months, so a 12-month lease starting March 15 ends on March 14 of the following year (or March 15 if the lease is "through" that date).
Why does February have 28 or 29 days?
February has 28 days in common years and 29 in leap years. This dates back to the Roman calendar reforms. A leap year occurs every 4 years, except for century years not divisible by 400. So 2000 was a leap year, but 1900 was not.
Can I calculate partial months as a decimal?
The calculator shows complete months plus remaining days. To get a decimal approximation, divide the remaining days by the number of days in that particular month. For example, 3 months and 15 days in a 30-day month equals approximately 3.5 months.