Calculation script needed for the number of months between a start date and end date.

This (free) tool should helps too : http://practicalpdf.com/the-practicalpdf-date-library-for-adobe-acrobat/

Date calculations are not simple in JavaScript. You can start by reading these tutorials:

https://acrobatusers.com/tutorials/working-with-date-and-time-in-acrobat-javascript

https://acrobatusers.com/tutorials/working-with-date-and-time-in-acrobat-javascript-part-2

https://acrobatusers.com/tutorials/working-date-and-time-acrobat-javascript-part-3-3

Just to clarify, the result should have at least 2 decimals. Here is the existing Excel calculation:

=DATEDIF(STARTDATE+2-DAY(STARTDATE+1),ENDDATE,"m")-(DAY(STARTDATE +1)-1)/DAY(EOMONTH(STARTDATE +1,0))+DAY(ENDDATE)/DAY(EOMONTH(ENDDATE,0))+0.03