iOS两个时间差

 NSDate *startDate = [NSDate date];
 [self is_mandatoryUpdate];// 执行这个方法所用的时间
 NSDate *endDate = [NSDate date];
NSCalendar *cal=[NSCalendar currentCalendar];
 unsigned int unitFlags = NSCalendarUnitYear| NSCalendarUnitMonth| NSCalendarUnitDay|NSCalendarUnitHour|NSCalendarUnitMinute|NSCalendarUnitSecond;
NSDateComponents *d = [cal components:unitFlags fromDate:startDate toDate:endDate options:0];
 NSLog(@"秒%ld",(long)[d second]);