Month Counter
Enter two dates to count the full calendar months between them, the leftover days, the span as decimal months (30.44-day average), and the total in days and weeks. Date order doesn't matter.
Example: with Start date 2025-01-01 · End date 2026-07-01 → Full months: 18 months, 0 days.
- Decimal months17.9 months (30.44-day average month)
- Calendar months touched19 calendar months touched (Jan 2025 to Jul 2026)
- Total days546 days (78 weeks, 0 days)
Computed by the calculator below using its default values. Change any input to see your own numbers.
A month here means a calendar month: Jan 15 to Feb 15 is one month whether that stretch is 28, 29, 30, or 31 days.
Three honest ways to count months
Full months use anniversary logic: the count ticks up each time the end date reaches the start's day-of-month. March 15 to July 4 is 3 full months (through June 15) plus 19 days. Decimal months divide total days by 30.4369 — the Gregorian average — which is the right measure for prorating rent or subscriptions. Calendar months touched counts every month bucket the span enters, which is what billing systems and month-labeled spreadsheets usually need.
Month-end dates are where counts get slippery. Jan 31 to Feb 28 shows 0 full months, 28 days here, because Feb 31 doesn't exist for the anniversary to land on. Some payroll systems round that to a month; this tool reports the strict calendar answer and lets you decide.
How it’s calculated
Full months = years × 12 + months from borrow arithmetic on (year, month, day); when the end day-of-month is smaller than the start's, one month is borrowed as the length of the month preceding the end date. Decimal months = total days ÷ 30.436875 (mean Gregorian month = 365.2425 ÷ 12). Months touched = 12 × year difference + month difference + 1. Total days = round((end − start) / 86,400,000 ms), both dates at noon.
Strict calendar counting; no rounding of near-months (Jan 31 to Feb 28 is 0 months, 28 days).
Months to days and weeks (average Gregorian month)
| Months | Days (avg) | Weeks (avg) |
|---|---|---|
| 1 | 30.4 | 4.3 |
| 3 | 91.3 | 13.0 |
| 6 | 182.6 | 26.1 |
| 12 | 365.2 | 52.2 |
| 18 | 547.9 | 78.3 |
| 24 | 730.5 | 104.4 |
Computed with 1 month = 30.436875 days (365.2425-day mean year ÷ 12); rounded to one decimal.
Common mistakes
- Assuming every month is 30 days — a 6-month span can be 181 to 184 days depending on where it starts.
- Confusing full months elapsed with calendar months touched: Mar 15 to Jul 4 is 3 full months but touches 5 month buckets.
- Letting month-end dates round up: Jan 31 to Feb 28 is not a full month by strict anniversary logic.
- Prorating with 30 days per month when the contract says calendar months — pick the convention the document uses.
Frequently asked questions
How do you calculate months between two dates?
Count anniversaries: months = 12 × (year difference) + (month difference), minus 1 if the end day-of-month hasn't reached the start's. The remainder in days comes from borrowing the length of the month before the end date.
Why does the decimal answer differ from the full-month count?
Full months follow the calendar's uneven month lengths, while decimal months divide total days by the 30.44-day average. An 18-full-month span of 546 days shows as 17.9 decimal months — both are correct measures of different things.
Is Jan 31 to Feb 28 one month?
Strictly, no: there is no Feb 31 for the anniversary to land on, so this tool reports 0 months, 28 days. Many billing systems treat month-end to month-end as one month — a convention, not calendar arithmetic.
How many months is 90 days?
About 2.96 average months (90 ÷ 30.4369). Depending on the actual months crossed, 90 days spans between 2 months 27 days and 3 months 2 days.