Convert UTC to local time with NSDateFormatter

Question 1

The question doesn't specify the nature of what you mean by converting, exactly, but the first thing you should do, regardless of the final goal, is to correctly parse the server response using a properly configured NSDateFormatter. This requires specification of the correct format string, and the time zone must be explicitly set on the formatter or it will infer it from the local time, which would be incorrect in most cases.

Specify The Format String

Let's look at the input string provided:

20140621-061250

This uses four digits for the year, two digits (with a zero-padding) for the month, and two digits (presumably, these will be zero-padded as well) for the day. This is followed by a -, then two digits to represent the hour, 2 digits for the minute, and 2 digits for the second.

Referring to the Unicode date format standards, we can derive the format string in the following way. The four digits representing the calendar year will be replaced with yyyy in the format string. Use MM for the month, and dd for the day. Next would come the literal -. For the hours, I assume that it will be in 24 hour format as otherwise this response is ambiguous, so we use HH. Minutes are then mm and seconds ss. Concatenating the format specifiers yields the following format string, which we will use in the next step:

yyyyMMdd-HHmmss

In our program, this would look like:

NSString *dateFormat = @"yyyyMMdd-HHmmss";

Configure the input date formatter

The time format above does not specify a time zone, but because you have been provided the specification for the server response that it represents the UTC time, we can code this into our application. So, we instantiate an NSDateFormatter, set the correct time zone, and set the date format:

NSTimeZone *inputTimeZone = [NSTimeZone timeZoneWithAbbreviation:@"UTC"];
NSDateFormatter *inputDateFormatter = [[NSDateFormatter alloc] init];
[inputDateFormatter setTimeZone:inputTimeZone];
[inputDateFormatter setDateFormat:dateFormat];

Convert the input string to an `NSDate`

For demonstration purposes, we hard-code the string you received from the server response; you would replace this definition of inputString with the one you get from the server:

NSString *inputString = @"20140621-061250";
NSDate *date = [inputDateFormatter dateFromString:inputString];

At this point, we have the necessary object to do any further conversions or calculations - an NSDate which represents the time communicated by the server. Remember, an NSDate is just a time stamp - it has no relation to a time zone whatsoever, which only plays a role when converting to and from string representations of the date, or representations of a calendrical date via NSDateComponents.

Next steps

The question doesn't clearly specify what type of conversion is needed, so we'll see an example of formatting the date to display in the same format as the server response (although, I can't think of a likely use case for this particular bit of code, to be honest). The steps are quite similar - we specify a format string, a time zone, configure a date formatter, and then generate a string (in the specified format) from the date:

NSTimeZone *outputTimeZone = [NSTimeZone localTimeZone];
NSDateFormatter *outputDateFormatter = [[NSDateFormatter alloc] init];
[outputDateFormatter setTimeZone:outputTimeZone];
[outputDateFormatter setDateFormat:dateFormat];
NSString *outputString = [outputDateFormatter stringFromDate:date];

Since I'm in UTC-06:00, printing outputString gives the following:

20140621-001250

It's likely you'll instead want to use setDateStyle: and setTimeStyle: instead of a format string if you're displaying this date to the user, or use an NSCalendar to get an NSDateComponents instance to do arithmetic or calculations on the date. An example for displaying a verbose date string to the user:

NSTimeZone *outputTimeZone = [NSTimeZone localTimeZone];
NSDateFormatter *outputDateFormatter = [[NSDateFormatter alloc] init];
[outputDateFormatter setTimeZone:outputTimeZone];
[outputDateFormatter setDateStyle:NSDateFormatterFullStyle];
[outputDateFormatter setTimeStyle:NSDateFormatterFullStyle];
NSString *outputString = [outputDateFormatter stringFromDate:date];

Printing outputString here gives us the following:

Saturday, June 21, 2014 at 12:12:50 AM Mountain Daylight Time

Note that setting the time zone appropriately will handle transitions over daylight savings time. Changing the input string to "20141121-061250" with the formatter style code above gives us the following date to display (Note that Mountain Standard Time is UTC-7):

Thursday, November 20, 2014 at 11:12:50 PM Mountain Standard Time

Summary

Any time you get date input in a string form representing a calendar date and time, your first step is to convert it using an NSDateFormatter configured for the input's format, time zone, and possibly locale and calendar, depending on the source of the input and your requirements. This will yield an NSDate which is an unambiguous representation of a moment in time. Following the creation of that NSDate, one can format it, style it, or convert it to date components as needed for your application requirements.

Question 2

To get your string into a NSDate, you would use a NSDateFormatter like this:

NSString *myDateAsAStringValue = @"20140621-061250"
NSDateFormatter *df = [[NSDateFormatter alloc] init];    
[df setDateFormat:@"yyyyMMdd-HHmmss"];
NSDate *myDate = [df dateFromString: myDateAsAStringValue];

You may want to read this post about working with Date and Time

EDIT:

To parse it as UTC you have to add the line:

[df setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"UTC"]];

Also, when you print it with NSLog, if you are using the same NSDateFormatter, you will get the input string as output (since you apply the inverse of the parsing function).

Here is the full code, for parsing and for getting the output with a standard format:

//The input
NSString *myDateAsAStringValue = @"20140621-061250";

//create the formatter for parsing
NSDateFormatter *df = [[NSDateFormatter alloc] init];
[df setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"UTC"]];
[df setDateFormat:@"yyyyMMdd-HHmmss"];

//parsing the string and converting it to NSDate
NSDate *myDate = [df dateFromString: myDateAsAStringValue];

//create the formatter for the output
NSDateFormatter *out_df = [[NSDateFormatter alloc] init];
[out_df setDateFormat:@"yyyy-MM-dd'T'HH:mm:ssz"];

//output the date
NSLog(@"the date is %@",[out_df stringFromDate:myDate]);

Question 3

One possible solution in Swift using NSDate extension (maybe it could help future viewers of this question):

import UIKit

// For your personal information: NSDate() initializer 
// always returns a date in UTC, no matter the time zone specified.
extension NSDate {
    // Convert UTC (or GMT) to local time
    func toLocalTime() -> NSDate {
        let timezone: NSTimeZone = NSTimeZone.localTimeZone()
        let seconds: NSTimeInterval = NSTimeInterval(timezone.secondsFromGMTForDate(self))
        return NSDate(timeInterval: seconds, sinceDate: self)
    }

    // Convert local time to UTC (or GMT)
    func toGlobalTime() -> NSDate {
        let timezone: NSTimeZone = NSTimeZone.localTimeZone()
        let seconds: NSTimeInterval = -NSTimeInterval(timezone.secondsFromGMTForDate(self))
        return NSDate(timeInterval: seconds, sinceDate: self)
    }
}