In a formula-type column, the MONTH_DELTA() function calculates the difference, in months, between two date-type columns, and returns a number. It is useful to measure a duration in months: subscription length, contract seniority, a payment schedule.

Syntax

Parameters

The output format of the column can be chosen in the output field options of the formula editor. For more details, see Output format options in the Formula Column article.

Examples

Calculate a difference in months

The column returns the number of months difference between the two dates. For the 2nd project, there is a 2 month difference between the End column and the Start column.

The formula subtracts the start month from the end month without considering the day. Example: start 01-01-2022, end 30-09-2022, MONTH_DELTA returns 8 although 9 would be expected. See the Counting months inclusively example below for this case.

Reading the sign of the result

The sign of the result indicates the direction of the difference, and both cases are useful depending on the need.

• Positive result: date_2 is later than date_1. Useful for elapsed duration, contract seniority, subscription length.
• Negative result: date_2 is earlier than date_1. Useful to identify a deadline already passed, for example MONTH_DELTA($Today, $Deadline).

Choose the order of the dates depending on what you want to read. To always get a positive difference regardless of order, place the older date first.

Counting months inclusively

By default, MONTH_DELTA() counts the difference between the month boundaries. For the period from 08-17-2026 to 11-20-2026, it returns 3. To count both the start month and the end month, add 1 to the result:

In this example: (8 − 11) + 1 = 4. The period indeed covers August, September, October, and November.

Difference spanning two years

If the period may span two years, use an explicit year-plus-month calculation. This formula adds the year difference converted to months and the month difference, then adds 1 to count both boundary months:

• (YEAR($End) - YEAR($Start)) * 12: converts the year difference to months.
• (MONTH($End) - MONTH($Start)): adds the month difference.
• + 1: counts both the start and end months.

Example, from 08-17-2025 to 11-20-2026: (2026 - 2025) * 12 + (11 - 8) + 1 = 12 + 3 + 1 = 16. The period indeed covers 16 months, from August 2025 through November 2026 inclusive.