Months Between Dates: 5 Mistakes to Avoid
Counts complete calendar months between two dates. The maths is not complicated, but a handful of errors come up again and again in this and related date & time calculations. Here is what to watch for.
When sharing dates internationally, write the month as a word to avoid confusion between day-month and month-day formats.
The Gregorian calendar has months of 28 to 31 days and adds a leap day almost every four years, which makes counting days by hand surprisingly error-prone. Converting dates to day numbers and subtracting avoids the problem.
1. Not deciding whether to include the end date
Counting inclusively adds one day.
2. Ignoring leap years
29 February changes day counts in leap years.
3. Mixing date formats
5/10 can mean 5 October or 10 May depending on the convention.
4. Overnight time errors
If a shift ends after midnight, add 24 hours before subtracting.
5. Subtracting times in decimal
9:45 minus 8:30 is 1 h 15 min, not 1.15 hours.
The correct method
Months = year difference × 12 + month difference, minus 1 if the end day is earlier.
For example, with these inputs:
- Start date: 15 Mar 2024
- End date: 5 Oct 2026
the calculator returns:
- Complete months: 30 months
Related calculations
- Age Calculator: Calculates exact age in years, months and days.
- Days Between Dates Calculator: Counts days between two dates.
- Add Days to a Date Calculator: Finds the date a number of days after another date.
- Subtract Days from a Date Calculator: Finds the date a number of days before another date.
The Months Between Dates Calculator applies the formula consistently, so it is a quick way to double-check your own working.
Run your own numbers: Months Between Dates Calculator