Crystal - manipulating date field when it is a string

I have a report that I'd like to have a second version of that runs by 3 months instead of 12. The report will prompt for the ending date, and will currently run based off of a range of 12 months prior. The prompt accepts the date as YYYYMM, ie 201801. It is a string variable, which makes calculations against it (like subtracting a year for the range) difficult. I could add in a starting date field, but I like for the report to figure everything out with minimal input. Getting a range for year is not so hard, just subtracting 100 from the entered value. Switching to a month range is more difficult though. I would need to subtract 2 from the input date, but not if it ends in 1 or 2. I would then need to subtract a year and a month or 2. I'm not sure how to do this though.