Converting any date to local date issue - objective-c

Question

I am stuck somewhere in NSDate and NSDateFormatter due to timezone issues.
I need to send time to server only in UTC(which is converted to unix time).
Here is my few steps what I am doing:
Selecting a date from the calender should be added with the current time and converted to UTC.
Compare the selected date with the current date. Just to know whether selected date is a past or future date. (Few other operations are to be done based on past/future/current date).
I have tried this code:
In a Category on NSDate :
-(NSDate *) toLocalTime{
NSDate* sourceDate = self;
NSTimeZone* sourceTimeZone = [NSTimeZone timeZoneWithName:#"UTC"];
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;
}
But the problem exists when I try to convert date to local, (at times I am not sure the current time is in local timezone or not). If they are in UTC then the above method works fine.
If the time is already in local timezone then it again adds the interval and I am getting incorrect time.
I am out of ideas, please help me.
Any idea will be highly appreciated.

Answer 1

NSDate represents the time since Jan 1st 1970 in UTC. Never try to pretend it is anything else. Never try to think of an NSDate as being in a particular local time.
So what you need is a date from a calendar + an offset which represents the time since midnight today.
To get today 0:00am UTC, you first need a Gregorian calendar for the UTC time zone.
NSTimeZone* utcTimeZone = [NSTimeZone timeZoneWithName:#"UTC"];
NSCalendar* gregorian = [[NSCalendar alloc] initWithCalendarIdentifier: NSGregorianCalendar];
[gregorian setTimeZone: utcTimeZone];
Now you use date components to get the hours, minutes and seconds since midnight UTC
NSUInteger unitFlags = NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;
NSDate *date = [NSDate date];
NSDateComponents *comps = [gregorian components: unitFlags fromDate:date];
If you have a date that is midnight UTC on the date from your calendar, you can get midnight UTC + your hours, minutes and seconds like this:
NSDate* theDateIWant = [gregorian dateByAddingComponents: comps
toDate: midnightUTCDateFromCalendar
options: 0];
NSLog(#"The final date is %#", theDateIWant);