[objc] NSDate 날짜비교

if ([date1 compare:date2] == NSOrderedDescending){

}

* NSOrderedSame : 같을때

* NSOrderedDescending :  date1가 date2보다 이후

* NSOrderedAscending :  date1이 date2보다 이전

http://stackoverflow.com/questions/5965044/how-to-compare-two-nsdates-which-is-more-recent