Question

I found the codes to calculate days difference between two dates here.
I write a method :

-(NSInteger)daysWithinEraFromDate:(NSDate *) startDate toDate:(NSDate *) endDate
{
    NSCalendar *gregorian = [[NSCalendar alloc]
                             initWithCalendarIdentifier:NSGregorianCalendar];
    NSInteger startDay=[gregorian ordinalityOfUnit:NSDayCalendarUnit
                                            inUnit: NSEraCalendarUnit forDate:startDate];
    NSInteger endDay=[gregorian ordinalityOfUnit:NSDayCalendarUnit
                                          inUnit: NSEraCalendarUnit forDate:endDate];
    return endDay-startDay;
}

This method has a problem: it can’t consider the timezone thing. Even I add a line like this:

[gregorian setTimeZone:[NSTimeZone localTimeZone]];

My test code is like this:

    NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
    [dateFormat setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSString *strDate = @"2012-09-03 23:00:00";
    NSDate *dateStart = [dateFormat dateFromString:strDate];
    strDate = @"2012-09-04 01:00:00";
    NSDate *dateEnd = [dateFormat dateFromString:strDate];
    NSLog(@"Days difference between %@ and %@  is:  %d days",[dateFormat stringFromDate:dateStart],[dateFormat stringFromDate:dateEnd],[self daysWithinEraFromDate:dateStart toDate:dateEnd]);

The result is:

Days difference between 2012-09-03 23:00:00 and 2012-09-04 01:00:00 is: 0 days

I want to get 1 day as result by the number of midnights between the two dates. My timezone is GMT +8. But this calculation is based on GMT, so I get the wrong days number. Is there anyway to solve this problem? Thank you.

---

Scott Lemmon‘s method can solve my problem. I rewrite my code like this:

-(NSInteger)daysWithinEraFromDate:(NSDate *) startDate toDate:(NSDate *) endDate
{
    NSCalendar *gregorian = [[NSCalendar alloc]
                             initWithCalendarIdentifier:NSGregorianCalendar];
    [gregorian setTimeZone:[NSTimeZone localTimeZone]];
    NSDate *newDate1 = [startDate dateByAddingTimeInterval:[[NSTimeZone localTimeZone] secondsFromGMT]];
    NSDate *newDate2 = [endDate dateByAddingTimeInterval:[[NSTimeZone localTimeZone] secondsFromGMT]];

    NSInteger startDay=[gregorian ordinalityOfUnit:NSDayCalendarUnit
                                            inUnit: NSEraCalendarUnit forDate:newDate1];
    NSInteger endDay=[gregorian ordinalityOfUnit:NSDayCalendarUnit
                                          inUnit: NSEraCalendarUnit forDate:newDate2];
    return endDay-startDay;
}

Answer 1

If the time zone offset isn’t working, how about just add or subtract it manually instead?

In your case NSDate *newDate = [oldDate dateByAddingTimeInterval:(-8 * 60 * 60)]; to subtract off your +8 hours.

Or if you want to find the GMT offset automatically as well then it would simply be NSDate *newDate = [oldDate dateByAddingTimeInterval:(-[[NSTimeZone localTimeZone] secondsFromGMT])

Another thought:
A perhaps easier solution would be to just disregard the time information altogether. Just set it to the same arbitrary number for both dates, then as long as the dates come from the same timezone you will always get the correct number of mid-nights between them, regardless of GMT offset.