Add Months & Month Difference Calculator
＋More features: contract renewal dates and a baby age-in-months list
Build a list of contract renewal dates
Uses the “base date” above as the contract start date and lists the renewal dates and cancellation-notice deadlines (end of month adjusted automatically).
Baby age-in-months date list (monthly calendar)
Uses the “base date” above as the birth date and lists the dates at 1 month old, 2 months old, and so on.
Just enter a date to instantly get the date N months later or how many months apart two dates are. End-of-month cases are adjusted automatically — one month after January 31 is February 28. Useful for checking contract periods and for a baby's age in months.
How to use
- Choose “Date N months later” or “Months between two dates”.
- Enter the base date and the number of months (or the end date). Use a negative number for months earlier.
- The result appears right below as you type — no button needed.
How it is calculated
- N months later: the month is shifted and the day is kept. If the target month has no such day, the date is adjusted to the end of the month (January 31 + 1 month → February 28, or February 29 in a leap year).
- Months between: counted as full months. If the day of the month has not yet been reached, one month is subtracted, and the remainder is shown in days (the same way a baby's age in months is counted).
- Example: January 15 to July 24 → 6 full months and 9 days
Sources and assumptions
- Arithmetic based on the Gregorian calendar. The end-of-month adjustment follows the same idea as period calculation under the Japanese Civil Code (when there is no corresponding day, the period ends on the last day of the month).
Assumptions and notes
- Some contracts count months differently (for example, including the first day). For important deadlines, check the contract clauses.
- The result differs from prorated calculations that treat one month as 30 days.
Frequently asked questions
What is the date one month after January 31?
February has no 31st, so it becomes the end of the month: February 28 (or 29 in a leap year). Period calculation under the Japanese Civil Code treats it the same way.
How is a baby's age in months counted?
It is counted in full months from the date of birth. A baby born on May 10 turns 1 month old on June 10. Enter the birthday and today in this tool's “months between” mode to find it.
Is “3 months later” the same as “90 days later”?
No. Because months have different lengths, 3 months later is 89–92 days later. For day-based calculations, use a day counter instead.