Try to convert the number of months to actual number of days, then add to your initial date, then take away 1. for example, if your date is on A1 (i.e. 01JAN05) and your number of days is in B1 (i.e. 60), you could type this in c1:
=sum(a1+b1-c1)
Ensure that A1 and C1 are formatted as dates and that b1 is formatted as number.
Let me know if it works!