Formula
A month is counted when the end day has reached the start day:
Months = calendar months from the start date to the end date
This is not days divided by 30. The weeks page is the day gap divided by 7.
Assumptions
- The end is on or after the start.
- Month lengths follow the calendar.
Worked example
January 15, 2026 to April 14, 2026 is 2 months, 30 days.
Input definitions
- Calendar span
- Years, months, and days, the same way an age is counted.
Limitations
A month here is a calendar month, not a 30-day block.
Questions
Why is January 15 to April 14 not 3 months?
April 14 has not reached the 15th, so the third month is not complete.
Is this an age?
The same calendar count is used. The age difference page is for two birthdays.