Depends what you want to work out - if you want 6 months in terms of 180 days ago, or 6 months in terms of 183 days ago (if you know what I mean). What I always do is convert the date to a base date, subtract the estimated number then convert back to a normal format.
If you wanted to be more accurate you'd calculate what months you went back and the number of days in them, then subtract from the base date.