Is there any simple functions out there that will allow you to add or subtract to a date and maintain the day without adjusting for months with 31, 30 29, or 28 days to throw off the day?

I need to substract 12 months (or a specific number of months) from a date and compare to another and do until it is lower than the other, but I need it to maintain the change date's day and move back whole months and roll the year as it passes. I can write a little function to do this, but I don't want to re-invent the wheel if there is something already out there which does this.