Why does NSDateComponents give me a different date? - ios

Question

Here's the code:
NSDate *dateLocalNow = [self getLocalDate:[NSDate date]];
NSLog(#"%#",dateLocalNow);
NSDateComponents *dateToCheckAgainst = [[NSCalendar currentCalendar] components:NSMinuteCalendarUnit | NSYearCalendarUnit|NSMonthCalendarUnit|NSDayCalendarUnit|NSMinuteCalendarUnit fromDate: dateLocalNow];
NSLog(#"%#", dateToCheckAgainst);
getLocalDate gives me the date in the local timezone. If the NSLog for dateLocalNow outputs:
2014-10-29 01:01:55 +0000
Here's getLocalDate source:
-(NSDate *)getLocalDate:(NSDate *)date {
NSDate* sourceDate = date;
NSTimeZone* sourceTimeZone = [NSTimeZone timeZoneWithAbbreviation:#"GMT"];
NSTimeZone* destinationTimeZone = [NSTimeZone systemTimeZone];
NSInteger sourceGMTOffset = [sourceTimeZone secondsFromGMTForDate:sourceDate];
NSInteger destinationGMTOffset = [destinationTimeZone secondsFromGMTForDate:sourceDate];
NSTimeInterval interval = destinationGMTOffset - sourceGMTOffset;
NSDate* destinationDate = [[NSDate alloc] initWithTimeInterval:interval sinceDate:sourceDate];
return destinationDate;
}
Why does NSLog for dateToCheck against give me the following?:
Calendar Year: 2014
Month: 10
Leap month: no
Day: 28
Minute: 1

Answer 1

An NSDate is independent of time zone. Internally, it stores the number of seconds that have elapsed since a reference point in time. That reference point has many human-readable labels. Here are a few human-readable labels for Cocoa's standard reference date:
1/1/01, 12:00:00 AM GMT
12/31/00, 6:00:00 PM CST
1/1/01, 4:00:00 AM GMT+4
These labels all represent the same instant in time, but that instant can be labeled in many different ways. There is only one way to represent that instant as an NSDate.
You've made a new NSDate that is the original NSDate, adjusted by some time zone offsets, but the iOS SDK doesn't know or care. It considers your new NSDate to be an instant in time based on an offset from its standard reference date. That new instant is different from your original instant (unless your system time zone happens to be GMT). You shouldn't expect it to produce the same result when converted to a human-readable string or an NSDateComponents, unless you set the time zone on your NSDateFormatter or NSCalendar just right—which you didn't do in your posted code.
So what do you do about this? You don't try to create an NSDate that is offset from another NSDate based on time zone offsets. Instead, you specify the time zone when you are converting an NSDate to a human-readable string, by setting the timeZone property of your NSDateFormatter. Or you set the timeZone property of your NSCalendar when you ask it for the components of the date. If you're constructing a date from components, you can also set the timeZone of the NSDateComponents before using the calendar to convert the components to an NSDate.
Thus:
#import <Foundation/Foundation.h>
int main(int argc, const char * argv[])
{
#autoreleasepool {
NSDate *now = [NSDate date];
NSDateFormatter *f = [[NSDateFormatter alloc] init];
f.dateStyle = NSDateFormatterShortStyle;
f.timeStyle = NSDateFormatterLongStyle;
f.timeZone = [NSTimeZone systemTimeZone];
NSLog(#"from formatter = %#", [f stringFromDate:now]);
NSCalendar *calendar = [NSCalendar currentCalendar];
calendar.timeZone = [NSTimeZone systemTimeZone];
NSDateComponents *components = [calendar components:NSMinuteCalendarUnit | NSYearCalendarUnit|NSMonthCalendarUnit|NSDayCalendarUnit fromDate:now];
NSLog(#"components = %#", components);
}
return 0;
}
Output:
2014-10-29 00:24:19.762 commandline[28543:303] from formatter = 10/29/14, 12:24:19 AM CDT
2014-10-29 00:24:19.763 commandline[28543:303] components = <NSDateComponents: 0x1001027a0>
Calendar Year: 2014
Month: 10
Leap month: no
Day: 29
Minute: 24

Answer 2

Can you step back and tell us what you're trying to do? This routine feels like you're trying to take [NSDate date] and "convert" it to local time. But that's not how dates work in Cocoa. NSDate doesn't have a concept of timezone. Only date formatters (and calendars) do.
So, when you get [NSDate date], it retrieves the current time. Yes, if you NSLog it, it may show it to you in GMT, but if you use a NSDateFormatter with the default timezone or grab components from a NSCalendar, it will always be in the local timezone (unless you override it as something else), with no adjustment needed.

Answer 3

+0000 means it is the GMT reperesentation of the local date.Use NSDateFormatter to log the display date and it will give the components displayed