Convert GMT NSDate to device's current Time Zone - iphone

Question

I'm using Parse.com to store some values:
These are GMT values. How do I convert these to the device's current time zone and get NSDate as a result?

Answer 1

NSDate is always represented in GMT. It's just how you represent it that may change.
If you want to print the date to label.text, then convert it to a string using NSDateFormatter and [NSTimeZone localTimeZone], as follows:
NSString *gmtDateString = #"08/12/2013 21:01";
NSDateFormatter *df = [NSDateFormatter new];
[df setDateFormat:#"dd/MM/yyyy HH:mm"];
//Create the date assuming the given string is in GMT
df.timeZone = [NSTimeZone timeZoneForSecondsFromGMT:0];
NSDate *date = [df dateFromString:gmtDateString];
//Create a date string in the local timezone
df.timeZone = [NSTimeZone timeZoneForSecondsFromGMT:[NSTimeZone localTimeZone].secondsFromGMT];
NSString *localDateString = [df stringFromDate:date];
NSLog(#"date = %#", localDateString);
// My local timezone is: Europe/London (GMT+01:00) offset 3600 (Daylight)
// prints out: date = 08/12/2013 22:01

Answer 2

The easiest method I've found is this:
NSDate *someDateInUTC = …;
NSTimeInterval timeZoneSeconds = [[NSTimeZone localTimeZone] secondsFromGMT];
NSDate *dateInLocalTimezone = [someDateInUTC dateByAddingTimeInterval:timeZoneSeconds];

Answer 3

This is a very clean way to change the NSDate to a local time zone date
extension NSDate {
func toLocalTime() -> NSDate {
let timeZone = NSTimeZone.local
let seconds : TimeInterval = Double(timeZone.secondsFromGMT(for:self as Date))
let localDate = NSDate(timeInterval: seconds, since: self as Date)
return localDate
}
}
taken from https://agilewarrior.wordpress.com/2012/06/27/how-to-convert-nsdate-to-different-time-zones/

Answer 4

Creating an Xcode test case like the following may help us remember the rules "forever":
- (void)test2015_05_23_07_07_07_Toronto {
NSString *utcDateString = #"2015_05_23 12:07:07";
NSDateFormatter *dateFormatter = [NSDateFormatter new];
dateFormatter.dateFormat = #"yyyy_MM_dd HH:mm:ss";
dateFormatter.timeZone = [NSTimeZone timeZoneWithName:#"UTC"];
NSDate *date2015_05_23_12_07_07_UTC = [dateFormatter dateFromString:utcDateString];
XCTAssertTrue([date2015_05_23_12_07_07_UTC.description isEqualToString:#"2015-05-23 12:07:07 +0000"]);
dateFormatter.timeZone = [NSTimeZone timeZoneWithName:#"EST"];
NSString *torontoDateString = [dateFormatter stringFromDate:date2015_05_23_12_07_07_UTC];
XCTAssertTrue([torontoDateString isEqualToString:#"2015_05_23 07:07:07"]);
// change format to add ZZZ
dateFormatter.dateFormat = #"yyyy_MM_dd HH:mm:ss ZZZ";
torontoDateString = [dateFormatter stringFromDate:date2015_05_23_12_07_07_UTC];
XCTAssertTrue([torontoDateString isEqualToString:#"2015_05_23 07:07:07 -0500"]);
XCTAssertEqual(1432382827, [date2015_05_23_12_07_07_UTC timeIntervalSince1970]);
NSCalendar *calendar = [NSCalendar autoupdatingCurrentCalendar];
calendar.timeZone = [NSTimeZone timeZoneWithName:#"EST"];
NSDateComponents *components = [calendar components:(NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond) fromDate:date2015_05_23_12_07_07_UTC];
XCTAssertEqual(2015, components.year);
XCTAssertEqual(5, components.month);
XCTAssertEqual(23, components.day);
XCTAssertEqual(7, components.hour);
XCTAssertEqual(7, components.minute);
XCTAssertEqual(7, components.second);
}

Answer 5

You could use a NSDateFormatter to achieve this result.
NSString *dateAsString = #"08/07/2013 04:06";
NSDateFormatter *df = [[NSDateFormatter alloc] init];
[df setDateFormat:#"dd/MM/yyyy HH:mm"];
NSTimeZone *gmt = [NSTimeZone timeZoneWithAbbreviation:#"GMT"];
[df setTimeZone:gmt];
NSDate *myDate = [df dateFromString: dateAsString];
NSLog(#"date: %#", myDate);

Answer 6

There are many answers to this question but I would recommend this one:
Convert GMT NSDate to device's current Time Zone
NSString *dateStr = #"2012-07-16 07:33:01";
NSDateFormatter *dateFormatter1 = [[NSDateFormatter alloc] init];
[dateFormatter1 setDateFormat:#"yyyy-MM-dd HH:mm:ss"];
NSDate *date = [dateFormatter1 dateFromString:dateStr];
NSLog(#"date : %#",date);
NSTimeZone *currentTimeZone = [NSTimeZone localTimeZone]; // <- Local time zone
NSTimeZone *utcTimeZone = [NSTimeZone timeZoneWithAbbreviation:#"UTC"];
NSInteger currentGMTOffset = [currentTimeZone secondsFromGMTForDate:date1];
NSInteger gmtOffset = [utcTimeZone secondsFromGMTForDate:date1];
NSTimeInterval gmtInterval = currentGMTOffset - gmtOffset;
NSDate *destinationDate = [[[NSDate alloc] initWithTimeInterval:gmtInterval sinceDate:date1] autorelease];
NSDateFormatter *dateFormatters = [[NSDateFormatter alloc] init];
[dateFormatters setDateFormat:#"dd-MMM-yyyy hh:mm"];
[dateFormatters setDateStyle:NSDateFormatterShortStyle];
[dateFormatters setTimeStyle:NSDateFormatterShortStyle];
[dateFormatters setDoesRelativeDateFormatting:YES];
[dateFormatters setTimeZone:[NSTimeZone systemTimeZone]];
dateStr = [dateFormatters stringFromDate: destinationDate];
NSLog(#"DateString : %#", dateStr);