For the OP, weeks are not relevant (FWIW, I'd countcalculate months, double that number, and check for partial months at beginning and end of the concerning period).

If payment were weekly, however, and retirement more than 6 years away, you would want to keep the correct amount of those additional weeks - or countcalculate "paydays" (Mondays, or whatever) which might start at a date different from January 1st, anyway.