iOS 比较时间 和 获取当前时间

// 获取系统当前的时间 
NSDate *date = [NSDate date]; 
NSDateFormatter *formatter = [[NSDateFormatter alloc] init]; 
[formatter setDateStyle:NSDateFormatterMediumStyle];
[formatter setTimeStyle:NSDateFormatterShortStyle]; 
[formatter setDateFormat:@"YYYY-MM-dd HH:mm:ss"]; 
NSString *nowTimeStr = [formatter stringFromDate:date]; 
NSLog(@"confromTimespStr = %@",nowTimeStr); 

//时间转为时间戳 
NSString *timeStr = @"2016-07-21 17:16:29"; 
NSDate *datenow = [formatter dateFromString:timeStr]; 
NSString *timeSp = [NSString stringWithFormat:@"%ld", (long)[datenow timeIntervalSince1970]];
NSLog(@"timeSp:%@",timeSp); //时间戳的值