NSDate and different timezones? - iphone

Question

Alright. Our application sends an NSString made out of the current user's username and todays date formatted in yyyyMMddHH. Our server, that is located in sweden, makes the exact same String and compares the two when it gets the call.
Now. We have realized, that if one of our users goes abroad, the timezone will change, resulting in complications.
if the iPhone user were to be in lets say, Seoul, South-korea. His NSString that is sent would be something like:
2011061718
Meanwhile, when our server gets the call, it will recreate its own datestring in this format because it's located in sweden.
2011061711
And therefore deny the user access to the functions on the server side.
To summarize:
How do i programmatically set a default static timezone in my application?
Atm we do this:
NSDate *aDate = [aDateFormatter stringFromDate:[NSDate date]];
And somehow we need the timezone and compare the difference between the user's actual timezone and change it to a swedish-timezone.
Any ideas?
EDIT: Ok. This application is only meant to be released in sweden. And we are using a combination of the user's username and the current date in the format of yyyyMMddHH to make a secure key, the key is meant to update itself whenever a new hour starts. The server, which is located in sweden in the timezone of GMT+1 makes a verification that the user is on an actual device using the application and not someone who has made a client of his own making soap-calls to our service.
Therefore, if one of our users goes outside the timezone, it will reject the user since the strings wont match.
This is why we want to set the default timezone for that function GMT+1 at all times. And this is what we're really looking for.
Thanks. Again.

Answer 1

If you need to make the client format the date, this is how to do it:
NSDate *now = [NSDate date];
NSCalendar *gregorianCalendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSTimeZone *timeZone = [NSTimeZone timeZoneWithName:#"Europe/Stockholm"]; // Sets the right time.
NSLocale *locale = [[NSLocale alloc] initWithLocaleIdentifier:#"en_US_POSIX"]; // Forces the date formatter to accept any format string.
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setCalendar:gregorianCalendar];
[formatter setTimeZone:timeZone];
[formatter setDateFormat:#"yyyyMMddHH"];
[formatter setLocale:locale];
NSString *dateString = [formatter stringFromDate:now];
NSLog(#"Date: %#", dateString);
[locale release];
[gregorianCalendar release];
[formatter release];
The date formatter will do all the work for you, you just need to configure it.
By setting the gregorian calendar, you're using the same calendar as in Sweden.
By setting the timezone, you'll get the time as it would be in Sweden.
By setting the locale to "en_US_POSIX" you make the formatter use the exact format you specify, and not append any AM/PM stuff.

Answer 2

Don't mess about with that. Send complete date time as a string and let the server figure out the times.
NSString *dtString = [[NSDate date] description];
This will create a string with the format
YYYY-MM-DD HH:MM:SS ±HHMM

Answer 3

Use -[NSDateFormatter setTimeZone:]. NSDate has nothing to do with it as it does not care about time zones.
Btw, your code will also fail if the user uses a different calendar.

Answer 4

Also a heads up, if you turn the AM/PM function on the dateformatter will pring out 'AM/PM' even if you dont place it in the format string.
To fix this also add an NSLocale to the DateFormatter.