How to use groovy script to find no of months between contract end date and contract start date

Summary:

We have a use case to use duration as a condition in our contract approval.

For example : 5 years or more will go to CEO, Less than 5 years will go to GM.

We create a DFF to store the number of months for duration. 60 or more will go to CEO, Less than 60 will go to GM.

However, we would like to use the system to calculation this duration using contract end date and contract start date in server script trigger in application composer.

For example: Start Date : 01-01-2026 End Date 31-12-2026, then duration is 12 months. How can we achieve this using groovy script?