Help with "how to calculate Expiration Date minus Effective Date +1 = NO. of reg months left"

[Former Community Member]

I just submitted a question, I thought...but here goes again - I am working on a form and need my Effective Date to be subtracted from my Expiration Date to give me a result in my NO. of reg months field.  (well I guess there are a couple questions in here....)

I have my Effective date as a date field MM/DD/YYYY and the Expiration Date is the same MM/DD/YYYY.  I need the expiration date (only 4 set dates allowed: 12/31/YYYY, 03/31/YYYY, 06/30/YYYY or 09/30/YYYY (would I do these 4 dates in a drop down or leave as a date field type??)) to subtract from the Effective date (either current or future (the days don't matter as it is not prorated daily just monthly)) and put the results in the: NO. of reg months field. 

For exp:  if a carrier wants to come on today or anytime in July (07/(any day)/2016 and he expires in 03/31/2017 then the calculation would give the results of 9 months in the numeric field for: NO. of reg months OR if he wanted to come on in August (08/any day/2016 then the results would come back as 8 months).  IF the carrier expired in 12/31/2016 then the results would be 5 or 6, etc.     Thanks, hopefully this is not too confusing.  Does the calculation field go in the NO. of reg months field?  DEEDEE HARRINGTON.   07/08/2016 posted