NSCalendar & NSDateComponents

一ヶ月前の今日を取得するには

NSCalendar *calendar = [[NSCalendar currentCalendar];
NSDateComponents *comps = [[[NSDateComponents alloc] init] autorelease];
comps.month = -1;
NSDate *date = [calendar dateByAddingComponents:comps toDate:[NSDate date]];

今月の1日を取得するには

NSCalendar *calendar = [[NSCalendar currentCalendar];
unsigned unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit;
NSDateComponents *comps = [calendar components:unitFlags fromDate:[NSDate date]];
comps.month = 1;
NSDate *date = [calendar dateFromComponents:comps];

こんな感じ。