You can have another approach :
Save a specific date in a field and then compute the month difference between that saved date and another date(which may be now or another time).
By this way you need not to increase those fields every month and also unpredicted happenings do not corrupt your computation plans.
So after saving your reference date use
DateDiff[
^] function to computed passed moths and that is the number that is you want.
Please see this page that described similar problem :
http://www.599cd.com/tips/access/dates-datediff-dateadd/?key=AllExperts[
^]
Hope it helps.
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