Ummm, OK, you made it work, but why? Why would you want to be able to call add_month() on a date and have it always represent the day before the last day of the month?
date d = August 30; // add six months. add_month(d); // September 30; add_month(d); // October 31? add_month(d); // November 30 add_month(d); // December 31? add_month(d); // January 30 add_month(d); // February 28/29 add_month(d); // March 28? 29? 30? 31? Just looking this there is no way to predict which day of the month the last call is going to be. I'd find it surprising that (d + year) != (12 * months) + d; I think you are looking for a simplistic function for a complex problem. IMO its a trap for the unwary and should be called out as such. I can see the need for LAST, but my contention is that its not enough information to do the least surprising thing. The easiest way to resolve this is to either write a FAQ, and or have a dumb rounding month adding function that can screw up the dates for those times when "meet you next month" is the right answer. Yours, -Gary-