A SAS numeric DATE variable is a count of the number of days since 1/1/1960. So, yes, you can add/subtract either using an assignment statement and/or using the INTNX function. My choice would be a combination of INTNX and using the WEEKDAY function, determine whether to add or subtract, since you mentioned the need for "closest" Monday. Personally, I don't see how the sameday operand helps in your quest, though.
An easy thing that you could try is to use 26 weeks. In intck, WEEK26.2 would get you 6 months starting on a Monday. You'll have to test it with different starting dates to see if the equivalency is adequate to the business need.