秒数

// NSDate —> NSString
[MHShortCut dateToString:@"yyyy-MM-dd" date:[NSDate date]];

// NSString -> NSDate
 NSDate *date = [MHShortCut stringToDate:@"yyyy-MM-dd HH:mm" dateString:self.endString];

// NSDate -> 毫秒数
[NSDate timeIntervalSince1970];

毫秒数 - > NSDate
[NSDate dateWithTimeIntervalSince1970: 毫秒数]