Month Calculator
Calculate months between two dates, add months to a date, or subtract months. Includes full breakdown in years, months, days and weeks — plus a quick-reference table and recurring event planner. Handles leap years automatically.
📅 Month Calculator
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months
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📋 Table of contents
How to calculate months between two dates
Counting the months between two dates requires careful handling of partial months and varying month lengths. Here is the standard algorithm:
Full months between dates:
months = (endYear − startYear) × 12 + (endMonth − startMonth)
IF endDay < startDay: subtract 1 (incomplete final month)
Example — Jan 15, 2023 → Oct 3, 2024:
months = (2024−2023) × 12 + (10−1) = 12+9 = 21
endDay (3) < startDay (15) → subtract 1
Full months = 20
Leftover days = days from Jan 15 → Feb 15 equivalent = 18 days
Decimal months (for billing/finance):
Partial month fraction = leftover days ÷ days in that month
Example: 18 days ÷ 31 days in Oct = 0.58
Total decimal months = 20.58
Why the partial month matters: Whether you include the partial month depends on context. Lease agreements typically count the move-in month as a full month. Loan amortisation uses decimal months. Subscription software often rounds up (ceiling) to bill the next full month.
How to add months to a date
Adding months to a date is straightforward except for one edge case: months have different lengths. The JavaScript Date object handles this automatically using "end-of-month clamping":
Standard case:
March 15 + 5 months → August 15 ✅ (August has 31 days)
End-of-month clamping:
January 31 + 1 month → February 28/29
(February doesn't have 31 days, so it clamps to last day)
March 31 + 1 month → April 30
(April has only 30 days)
Leap year example:
January 31, 2024 + 1 month → February 29, 2024
January 31, 2025 + 1 month → February 28, 2025
Financial convention: Contracts and billing software typically use end-of-month clamping. If a subscription starts on January 31, the next billing date is February 28/29 (not March 2). This is the international standard for monthly recurring charges.
Month lengths and leap years
| Month | Days | Quarter | Notes |
|---|---|---|---|
| January | 31 | Q1 | — |
| February | 28 or 29 | Q1 | 29 days in leap years |
| March | 31 | Q1 | — |
| April | 30 | Q2 | — |
| May | 31 | Q2 | — |
| June | 30 | Q2 | — |
| July | 31 | Q3 | — |
| August | 31 | Q3 | — |
| September | 30 | Q3 | — |
| October | 31 | Q4 | — |
| November | 30 | Q4 | — |
| December | 31 | Q4 | — |
A leap year occurs when the year is divisible by 4, except for century years (e.g. 1900) which must be divisible by 400 (so 2000 was a leap year, but 1900 was not). Leap years matter when calculating months that span February.
Common use cases for the month calculator
- Lease & rental agreements — Confirm the exact end date of a 12-month or 24-month lease. Verify how many months of rent have elapsed.
- Subscription billing — Calculate when a 6-month, 12-month, or 18-month subscription will renew. Find past billing dates for disputes.
- Loan amortisation — Determine how many months remain on a mortgage or car loan. Calculate the exact payoff month.
- Pregnancy milestones — Track gestational months (pregnancy is approximately 9 months / 40 weeks). Calculate due dates from LMP.
- Project planning — Break a multi-year project into monthly milestones. Calculate sprint end dates and quarterly reviews.
- Visa & permit duration — Many visa categories are granted in months. Calculate exact expiry dates and renewal windows.
- Warranty & guarantee periods — Determine exactly when a 12-month or 24-month warranty expires from purchase date.
- Age calculations — Determine someone's exact age in months (useful for infant development tracking and medical contexts).
FAQs
Full months = (End year − Start year) × 12 + (End month − Start month). Subtract 1 if end day is less than start day (incomplete final month). Example: Jan 15 to Oct 3 = 12 + 9 = 21, minus 1 because 3 < 15 = 20 full months. This calculator also shows leftover days, total days, and decimal months for billing purposes.
Add the months to the month number and adjust for year rollover. If adding 5 to March (month 3): 3+5=8 = August. If the result exceeds 12, roll over to the next year. Watch out for end-of-month clamping: adding 1 month to January 31 gives February 28 (or 29 in a leap year), not March 2.
Use the Add Months mode above — it defaults to today's date. Six months from today will always land on the same day-of-month, six calendar months later. If the target month has fewer days than the start day (e.g. adding 1 month to August 31 gives September 30, not October 1), the date clamps to the last day of the target month.
There are 12 months in a Gregorian calendar year. On average, a month is 30.44 days (365.25 ÷ 12). Months range from 28 days (February in non-leap years) to 31 days. Four months have 30 days (April, June, September, November), seven have 31 days, and February has 28 or 29 days.
Use the "Months Between" mode: enter the lease start date and end date. The calculator shows full months, leftover days, and total days. For example, a lease from April 1, 2024 to March 31, 2025 = exactly 11 months and 30 days (not 12 full months, because March 31 is one day before completing the 12th month). Most landlords round up; the calculator shows "Round up" mode for this.