If you're dealing with months and years you need something that knows how many days each month has and which years are leap years.
Enter the Gregorian Calendar (and other culture-specific Calendar implementations).
While Calendar doesn't provide methods to directly calculate the difference between two points in time, it does have methods such as
DateTime AddWeeks(DateTime time, int weeks)
DateTime AddMonths(DateTime time, int months)
DateTime AddYears(DateTime time, int years)
