Why there is more than one answer
Months are not all the same length, so there is no single correct number of months between two dates. The right method depends on what you are counting. This tool shows the common ones next to each other.
The methods
| Method | What it gives | Used for |
|---|---|---|
| Complete months and days | Whole months, then the days left over | Lease and contract length, subscriptions, age in months |
| Decimal months (calendar) | Whole months plus the part of the next month that has passed | Pro-rata amounts |
| Decimal months (average) | Days divided by 30.4375, the average length of a month | Quick estimates |
| 30/360 months | Every month has 30 days and a year has 360 | Interest and bond calculations |
| Calendar months touched | How many different months the period includes | Billing by calendar month |
Complete months
The tool adds whole months to the start date and stops before it passes the end date. From 15 January to 14 March is 1 month and 27 or 28 days, because the second full month ends on 15 March. When the start day does not exist in a month, such as the 31st, the last day of that month is used. This matches the whole months that the Excel function DATEDIF gives with the unit m.
30/360
The 30/360 method is used in finance. It treats every month as 30 days. The tool uses a simplified US form of the rule: a start day of 31 becomes 30, and an end day of 31 becomes 30 when the start day is 30 or more. Contracts specify the exact convention, so check yours.
Include the end date
Tick Include the end date to count the last day as part of the period. This adds one day before the months are worked out.
Related calculators
See date difference for years, months and days, days between dates for days, and weeks between dates. For percentages of a period use the percentage change calculator. The guide shows the formulas.