About the Months Between Dates Calculator
Many things are measured in months: pregnancies and babies' ages, rental agreements, phone contracts, subscriptions, probation periods, loan terms, warranties and the time since a medical treatment. But months are awkward units — they range from 28 to 31 days — so "how many months between these dates?" can have more than one reasonable answer.
This months between dates calculator gives all of them. It finds the whole calendar months plus leftover days, the span as a decimal number of months (two ways), the same span in years and months, and the number of calendar months the range touches, which is what matters for anything billed per calendar month.
How to Use the Months Between Dates Calculator
Enter the start date and the end date.
The number of months appears immediately, with the alternative measures.
How Months Are Counted
whole months: the most monthly anniversaries of the start date that fit
leftover days: days from the last anniversary to the end date
decimal (calendar): whole months + leftover days ÷ days in the next month
decimal (average): total days ÷ 30.436875 (365.2425 ÷ 12)
months touched: every calendar month the range falls in, counting both endsA start date on the 29th, 30th or 31st has anniversaries on the last day of shorter months.
Step-by-Step Example
From 15 January 2025 to 29 September 2026.
Anniversaries: 15 February 2025 … 15 September 2026 → 20 whole months
Leftover: 15 September → 29 September = 14 days
Next month: 15 September → 15 October = 30 days
Calendar decimal: 20 + 14 ÷ 30 = 20.47 months
Average decimal: 622 days ÷ 30.44 = 20.44 months
In years: 1 year, 8 months and 14 days
Months touched: January 2025 to September 2026 = 21The answer is 20 months and 14 days.
Month-End Start Dates
What is one month after 31 January? February has no 31st, so the anniversary falls on the last day of February. The calculator counts 31 January to 28 February 2026 as exactly one month, the same rule used by banks, spreadsheets and most contracts. Similarly, 31 August to 30 September is one month, and 31 March to 30 April is one month. This means a month measured from the end of a month can be as short as 28 days.
Which Answer Should I Use?
For ages and elapsed time, such as a baby's age or time since an event, use the whole months and days — "7 months and 12 days". For pro-rata calculations, such as splitting an annual fee or salary by months worked, use one of the decimal answers; the average-month method is simplest and treats every day equally. For billing by calendar month, such as rent or subscriptions charged on the first of each month, use the months touched, since a stay from 25 January to 5 February spans two billing months even though it lasts only 11 days.
Why the Decimal Answers Differ
The calendar method measures the leftover days against the actual length of the month that follows, so 14 days is 14/30 of a 30-day month but 14/31 of a 31-day month. The average method uses the same month length, 30.44 days, for every month. The two normally differ by only a few hundredths of a month, but for short spans that include February the gap can be larger. Neither is wrong; they simply answer slightly different questions.
Months in Pregnancy and Baby Ages
Pregnancy is usually counted in weeks rather than months precisely because months vary. A full-term pregnancy of 40 weeks is 280 days, a little over nine calendar months from the first day of the last period. For babies, age in months is standard up to about two years: doctors track development milestones and growth by months, and a baby born on 15 January is "six months old" on 15 July.
Contracts and Notice Periods
Contracts commonly specify periods in months: a 12-month tenancy, a 3-month notice period, a 24-month phone plan. These usually run from a date to the same date in a later month, with the end-of-month rule applied. A 3-month notice period starting on 30 November ends on 28 February (or 29 February in a leap year), not on 2 March. Always check how the contract defines a month, because some specify 30-day periods instead.
Converting Months to Other Units
Once you have the number of months, converting is simple: divide by 12 for years, so 20 months is 1.67 years, or 1 year and 8 months. Multiplying by 4.35 gives an approximate number of weeks, since an average month is 30.44 days, or 4.35 weeks. For an exact count of weeks or days, use the total days rather than multiplying months, because the average hides the variation between short and long months.
Understanding Your Result
The headline gives whole months and leftover days.
The decimal months line gives the calendar-based decimal.
The by average month line gives the average-month decimal.
The in years line splits the whole months into years and months.
The calendar months touched line counts the months the range falls in.
When Should You Use This Calculator?
Use it to find a baby's or child's age in months.
Use it to work out how long a contract, tenancy or subscription has run.
Use it for pro-rata fees, salaries and refunds.
Use it to count billing months for rent or services.
Common Mistakes
Dividing days by 30. It drifts over long spans; use the calendar or average method.
Forgetting the end-of-month rule. 31 January to 28 February is one full month.
Confusing months elapsed with months touched. Billing often uses the latter.
Assuming every contract uses calendar months. Some define a month as 30 days.
Mixing up the date order. The calculator swaps them, but check the range shown.