Handling Day light time difference

Question

As per our application specification, we need to schedule call for conference, while I scheduling I have been facing problem with Day light time difference.

This is the case: US day light time changes on Nov 3 this year, when I schedule a call for eg: 5 P.M on Nov 4, that time I am converting into GMT and send to server. If I convert that GMT time its giving 4 P.M Nov 4, instead of 5 P.M. Even though its correct only, however as per user perspective he scheduled for 5 P.M not 4 P.M, so how to handle this case.

I'm converting into GMT like ...

- (NSDateComponents*) convert_to_gmt_time : (NSDate*) date
{
    NSDate *localDate = date;
    NSTimeInterval timeZoneOffset = [[NSTimeZone defaultTimeZone] secondsFromGMT];
    NSTimeInterval gmtTimeInterval = [localDate timeIntervalSinceReferenceDate] - timeZoneOffset;
    NSDate *gmtDate = [NSDate dateWithTimeIntervalSinceReferenceDate:gmtTimeInterval];

    NSDateComponents *date_comp = [[NSCalendar currentCalendar] components: NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit fromDate:gmtDate];

    return date_comp;
}

Edit:

This is how I'm coverting GMT time to local...

  NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    [gregorian setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"GMT"]];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"MMM dd, yyyy"];
NSDate *strt_date_loc = [gregorian dateFromComponents:start_date]; // start_date i'm filling after received from server as GMT.

    // Get date string.
    NSString *strt_date = [dateFormatter stringFromDate: strt_date_loc];

Any help is really appreciated. Thanx.

Answer 1

Got the problem,

Problem was with GMT conversion, as I did consider the current time difference to GMT, and not the time difference that is in effect at the date to be converted.

This is how I fixed...

NSCalendar *cal = [NSCalendar currentCalendar];
[cal setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];
NSDateComponents *date_comp = [cal components: NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit fromDate:localDate];

Answer 2

Again and again and again and …: You cannot convert a date to a timezone. You can convert representations of dates to time zones. This does not change the date. Don't try to be more tricky than dates are.

Unexpected value from NSDate

If you have a date representation in local format (i. e. US) simply build a date from it by using a local date formatter. Then send this date to $anywhere.

On the way back read this date (not one of its representations) and build a local representation with a local formatter.

Never in the meaning of never send any local representation of a date to anywhere. GMT is a local representation. (What does stand G for?)

Answer 3

An NSDate is an absolute time, and not based upon a time zone. So basically it is like storing the time as described at GMT.

So you just need to get a calendar whose timezone is GMT, and then get the components.

 NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
 [calendar setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"GMT"]];

 NSDateComponents *date_comp = [calendar components: NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit fromDate:gmtDate];