IOS NSDateFormatter Problems

Question

We are building an app which requires the storage of a date for an entry on the device, the app will be international so we hit two dilema's / challenges.

1. User Preferences

   If the user chooses the 12 hour rahter than 24 hour format we are returned from [NSDate date] a date like this 2012-07-17 11:26:03 AM which for sorting in a SQLite database is less than optimal as we cannot store it as a date.

2. User Locale

   Typically this is ok however here in blighty we have a wonderfult thing called british summertime. which adds one hour every October 25th - 30th in a cycle and removes one hour every March 25 - 31th in a cycle so if no adjustment is made for 8 months of the year the time is one hour behind.

What I need to achieve is a consistent date formatted like this: 2012-07-17 11:26:03 no matter where the device is located and also taking into account where GMT+1 comes into place.

Any help would be awesome.

EDIT*

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
dateFormatter.dateFormat = @"yyyy-MM-dd'T'HH:mm";
NSTimeZone *gmt = [NSTimeZone timeZoneWithAbbreviation:@"GMT+01:00"];
[dateFormatter setTimeZone:gmt];
NSString *timeStamp = [dateFormatter stringFromDate:[NSDate date]];
NSDate *localDate = [dateFormatter dateFromString:timeStamp];


NSLocale* currentLocale = [NSLocale currentLocale]; 
NSString* countryCode = [currentLocale objectForKey:NSLocaleCountryCode];
NSLog(@"Country Code: %@", countryCode);
NSLog(@"Current Loacle: %@", currentLocale);
if(([countryCode isEqualToString: @"GB"])){
    NSDate *mydate = [NSDate date];
    NSDate *fiddlyFoo = [mydate dateByAddingTimeInterval:3600];
    NSLog(@"Print GMT +1 %@",fiddlyFoo); 
} else {
    NSLog(@"Print GMT Local   %@",localDate); 
}

Answer 1

I'm doing something like this now. Note that NSDate "knows" about the current timezone and daylight savings time etc. So you just need to get the GMT version of the time in a sortable representation. I'd suggest RFC 3339 but you can use variations on it per your needs:

This code:

NSCalendar *gregorian = [[NSCalendar alloc]initWithCalendarIdentifier:NSGregorianCalendar];

// Create a local date for London, for testing purposes
NSDateComponents *comps = [NSDateComponents new];
[comps setTimeZone:[NSTimeZone timeZoneWithName:@"Europe/London"]];
[comps setDay:1];
[comps setMonth:7];
[comps setYear:2012];
[comps setHour:14];
NSDate *date = [gregorian dateFromComponents:comps];

// Just want to show this date is 2PM in London July 1st
NSDateFormatter *curFormat = [NSDateFormatter new];
[curFormat setDateStyle:NSDateFormatterFullStyle];
[curFormat setTimeStyle:NSDateFormatterFullStyle];
NSLog(@"Reference date is good no: %@", [curFormat stringFromDate:date]);

// So now we get the date as a rfc3339 string, referenced to GMT
NSString *timeString;
{
    NSDateFormatter *rfc3339 = [NSDateFormatter new];
    [rfc3339 setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss'Z'"];
    rfc3339.timeZone = [NSTimeZone timeZoneWithName:@"UTC"];
    timeString = [rfc3339 stringFromDate:date];
}
// referenced to UTC (sortable with any other time), can save in SQL DB etc
NSLog(@"Date as rfc3339 string: %@", timeString);

// Now lets convert it back into a BST time
NSDate *newDate;
{
    NSDateFormatter *rfc3339 = [NSDateFormatter new];
    [rfc3339 setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss'Z'"];
    rfc3339.timeZone = [NSTimeZone timeZoneWithName:@"UTC"];
    newDate = [rfc3339 dateFromString:timeString];

    // we want to show this as a string 2012-07-17 11:26:03
    NSDateFormatter *newFormatter = [NSDateFormatter new];
    [newFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"]; // local time
    [newFormatter setTimeZone:[NSTimeZone timeZoneWithName:@"Europe/London"]];

    NSLog(@"Local string using 24 hour clock: %@", [newFormatter stringFromDate:newDate]);
}

Generates this output, which I believe is what you want:

TimeTester[58558:f803] Reference date is good no: Sunday, July 1, 2012 9:00:00 AM Eastern Daylight Time
TimeTester[58558:f803] Date as rfc3339 string: 2012-07-01T13:00:00Z
TimeTester[58558:f803] Local string using 24 hour clock: 2012-07-01 14:00:00