Get number of days between two NSDate dates in a particular timezone

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;
}

Scott Lemmon · Accepted Answer · 2012-09-04T07:12:43.643

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.

Hi, thank you for your answer. I just thought about disregarding the time information. I have done this in my codes. But your first solution is brilliant! I think it's a more precise way to solve this problem. I have try it in my code, I will show them. — Vigor, Sep 04 '12 at 07:18

score 3 · Answer 2 · edited May 23 '17 at 12:34

What you really want is the NSDate method timeIntervalSinceDate:, and take that result and if it's more than 0 but less than 86400 (the number of seconds in a day), that's one day. Otherwise, divide your result by 86400 and you'll get the number of days.

The way you currently have your code, there's only 2 hours between the two days and that's why you are seeing a result of 0 and not one.

Edit - and to determine if midnight has happened, let's try this function I just wrote off the top of my head:

- (NSDate *) getMidnightDateFromDate: (NSDate *) originalDate
{
    NSDateComponents *components = [[NSCalendar currentCalendar] components:NSIntegerMax fromDate:originalDate];
    [components setHour:0];
    [components setMinute:0];
    [components setSecond:0];
    NSDate *midnight = [[NSCalendar currentCalendar] dateFromComponents:components];
    return(midnight);
}

- (BOOL) howManyDaysDifferenceBetween: startDate and: endDate
{
    NSDate * firstMidnight = [self getMidnightDateFromDate: startDate];
    NSDate * secondMidnight = [self getMidnightDateFromDate: endDate];
    NSTimeInterval timeBetween = [firstMidnight timeIntervalSinceDate: secondMidnight];

    NSInteger numberOfDays = (timeBetween / 86400);

    return(numberOfDays);
}

which I'm basing off Dave Delong's answer to this question. No guarantees that my code will work (I didn't test it), but I think the concept is sound.

Thank you for your answer. I want to determine if two NSDate instances are in different dates by the number of midnights between the two dates. Under your method, I still will get 0 day. But 2012-09-03 and 2012-09-04 are in different dates, even though the interval is only 2 hours. If I change the second date to 2012-09-04 08:00:00, I will get 1 day as result under my method. I think it's because the timezone. — Vigor, Sep 04 '12 at 06:54

Get number of days between two NSDate dates in a particular timezone

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