iOS中，获取到当前的时间，减去1970年的时间差，换算成秒怎么做

01 //获取当前时间
02 NSDate *now = [NSDate date];
03 NSLog(@”now date is: %@”, now);
04
05 NSCalendar *calendar = [NSCalendar currentCalendar];
06 NSUInteger unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;
07 NSDateComponents *dateComponent = [calendar components:unitFlags fromDate:now];
08
09 int year = [dateComponent year];
10 int month = [dateComponent month];
11 int day = [dateComponent day];
12 int hour = [dateComponent hour];
13 int minute = [dateComponent minute];
14 int second = [dateComponent second];