Months Between Dates

How it is worked out

months = month boundaries crossed, then the days left over

Why it is done this way

Months are the awkward calendar unit, because they are not all the same length. Counting them properly means stepping from the start date to the same day of each following month until you overshoot; the number of steps is the months and what remains is the days.

Step by step

1. Step forward one month at a time from the start date
2. Stop when the next step would pass the end date
3. The number of steps is the whole months
4. The days between there and the end date are the remainder

What is worth knowing

The step-by-step method is what makes 31 January plus one month land on 28 February rather than 3 March, which is the convention almost every legal and financial system uses. Dividing total days by 30.44 gives an approximate figure that is fine for a rough sense of scale and wrong for anything that turns on a date. The gap matters most over short periods: two dates 60 days apart can be one month and 29 days or two months exactly, depending entirely on which months they cross.

From one month to another, how many months

Counting the month boundaries crossed within the same year. If you need to include both end months, add one.