As a point of reference, the ADD_MONTHS function in Oracle would return the last day of February in that situation. Subtracting a month from March 30 would also return February 29.

I think that approach makes the most sense. The programmer can choose whether or not to verify the dates before adding/subtracting months (if an error were returned, the programmer would be forced to check each date). And, I agree with Blue that adding one month should always return a date in the next month, and should not wrap over to the month after that.