Month Calculator | Month Counter

Created by Tibor Pal, PhD candidate
Reviewed by Dominik Czernia, PhD candidate and Jack Bowater
Last updated: Mar 10, 2022

This month calculator, or month counter, is a simple tool to compute the precise number of months between two given dates. In case if the number of months between the two dates is incomplete, the number of days is given instead.

So if you would like to know how many months are between two dates, you are in the right place!

How to use the month calculator? - how many months between two dates?

After entering the start date and the end date, the month calculator will return the number of months and any remaining days.

An essential feature of the calculator is that it takes into account the precise number of days of all of the months, including the effect of leap years.

What is a leap year?

Do you know anyone who has a birthday on the 29th February? If you do, or if by chance you are the "lucky person" in question, you must know what a leap year is.

A leap year (also known as an intercalary year or bissextile year) is a calendar year that has one extra day added to keep the calendar year harmonized with the astronomical year or seasonal year.

Since the complete orbit of the Earth around the Sun takes approximately 365 days, 5 hours, 48 minutes, 45 seconds, a calendar that has only whole days in it will drift over some time. By adding (or intercalating with a technical terminology) an extra day, the difference can be eliminated. A year that is not a leap year is a common year.

When is a leap year?

So when is leap year? In the Gregorian calendar, most years that are divisible by four are leap years. In each leap year, the month of February has 29 days instead of 28. The extra day every four years compensates for the fact that the solar year is lightly longer than 365 days. The duration of a solar year is slightly less than 365.25 days (or, more precisely, it is 365.24219 days long).

However, we must still do some further correction. If a year is exactly divisible by four, it is a leap year, except for years that are precisely divisible by 100. The exceptions are years that are exactly divisible by 400. For instance, the years 1700, 1800, and 1900 are common years, but the years 1600 and 2000 are leap years.

Translating all of the above into an algorithm, which is applied in this calculator as well, the following conditions are required to determine if a year is a leap years:

if (year is not divisible by 4) then (it is a common year)
else if (year is not divisible by 100) then (it is a leap year)
else if (year is not divisible by 400) then (it is a common year)
else (it is a leap year)

Tibor Pal, PhD candidate
Starting date
Ending date
The number of full month(s) between the chosen dates is precisely 12 month(s).
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