You probably first need to be specific as to what precisely constitutes a 'month'? eg 30 days? 31 days? 28 (or 29?!) days? or 4 weeks? or whatever the same day # in the next month, etc. This will directly influence the complexity of the solution required.
You should also consider (all!) potential edge cases. eg what is "2 months" after Dec 31?
FWIW, if you can make a 'month' be 4 weeks/28 days, the solution can be a simple calculation. Otherwise, perhaps see Calculate age in years, months and/or days for, um, 'inspiration' 