Month CalculatorCount the months between dates, or add and subtract months
Count the complete months between two dates (plus leftover days and decimal months), or add or subtract months from a date, using the same month-end rule as Excel’s EDATE.
- Matches Excel EDATE/DATEDIF
- Month-end rule handled
- Decimal months
- Month block view
Month calculator
Months between
—
How do you count the months between two dates?
Count the complete calendar months from the start date, then the leftover days. A month is complete when you reach the same day number in a later month, or that month’s last day if it is shorter. From March 31, 2026 to January 15, 2027 there are 9 complete months and 15 days (about 9.5 months). To add months, keep the day number: January 31 + 1 month = February 28, because February has no 31st.
Months to days and weeks
Months vary from 28 to 31 days. These averages use the Gregorian mean month of 30.436875 days (365.2425 ÷ 12).
| Months | Avg. days | Avg. weeks | Years |
|---|---|---|---|
| 1 | 30.4 | 4.3 | 0.08 |
| 2 | 60.9 | 8.7 | 0.17 |
| 3 | 91.3 | 13.0 | 0.25 |
| 4 | 121.7 | 17.4 | 0.33 |
| 6 | 182.6 | 26.1 | 0.5 |
| 9 | 273.9 | 39.1 | 0.75 |
| 12 | 365.2 | 52.2 | 1 |
| 18 | 547.9 | 78.3 | 1.5 |
| 24 | 730.5 | 104.4 | 2 |
| 36 | 1,095.7 | 156.5 | 3 |
| 48 | 1,461.0 | 208.7 | 4 |
| 60 | 1,826.2 | 260.9 | 5 |
| 120 | 3,652.4 | 521.8 | 10 |
Month math that catches people out
“1 month” ≠ 30 days
Between February 1 and March 1 there are 28 days, and between July 1 and August 1 there are 31. Contracts, leases and prescriptions say which unit they mean.
Month-end dates don’t round-trip
January 31 + 1 month = February 28, but February 28 + 1 month = March 28, not March 31.
Six months isn’t half a year in days
Six months can be 181 to 184 days. Use the half birthday calculator if you need an exact midpoint.
Pregnancy months
Pregnancies are tracked in weeks (40), not months, because “9 months” is ambiguous.
Related calculators
Months are one unit. Switch to another when it fits better:
Manzoor Ahmad
Calculator Developer · Pharmacist · Founder of DexoCalc
I’m a pharmacist turned web developer with 13+ years of experience building web tools. Month arithmetic causes real errors with prescriptions and leases, so this calculator follows the same rules as Excel’s EDATE and DATEDIF, and I checked it against them for every month-end case.
Published · Last reviewed · Author profile · Report an issue
Month calculator: frequently asked questions
Enter both dates. The calculator counts complete calendar months plus the leftover days, and also shows decimal months, total days and weeks.
Switch to ‘Add / subtract months’, enter today’s date and 6. The day number stays the same unless that day doesn’t exist in the target month.
February 28, or February 29 in a leap year. When the target month is shorter, the result moves to its last day. This is the same rule used by Excel’s EDATE function.
30.436875 days, which is 365.2425 ÷ 12. Actual months have 28, 29, 30 or 31 days.
About 2.96 months on average, or roughly 3 months. The exact count depends on which months the 90 days cover.
Yes. Complete months match =DATEDIF(start,end,”m”), and adding months matches =EDATE(start, n).
Sources & references
- EDATE function, Microsoft Support
- DATEDIF function, Microsoft Support
- ISO 8601: Date and time format, International Organization for Standardization
Calendar arithmetic only. Check how your contract or lease defines a month.