0

I want to display the following string on my time axis:

"GMT/BST"

Here's the code:

 NSDateFormatter *dateformatter=[[NSDateFormatter alloc] init];
[dateformatter setDateFormat:@"zzz"];
timeZoneString = [NSMutableString stringWithFormat:@"%@ / %@",[dateformatter stringFromDate:startDate],[dateformatter stringFromDate:endDate]];

But this gives "GMT/GMT+01:00"

What is the NSDateFormatter code to turn "GMT+01:00" into "BST" ? I can't get the right formatters to do this, having tried z|zzz|Z|ZZZ|v|V see... http://waracle.net/iphone-nsdateformatter-date-formatting-table/

Nick T
  • 897
  • 8
  • 30
  • Well, if `knownTimeZoneNames` doesn't show the name you want, it's probably not in /usr/share/zoneinfo. – Hot Licks Feb 07 '14 at 16:56
  • `Europe/London` is the closest in `knownTimeZoneNames`. I suppose I could programmatically change string to `BST` if = `GMT+01:00` . But that seems a bit of a hack. – Nick T Feb 07 '14 at 17:24
  • (What happens if you attempt to set the timezone to "GMT/BST"?) – Hot Licks Feb 07 '14 at 17:26

2 Answers2

2

Turns out there is a built in array of 48 time zone abbreviations (e.g. 'BST') in iOS. 

NSDictionary *tzDict = [NSTimeZone abbreviationDictionary];

There is an array of 419 time zone names in this array (e.g. 'Europe/London'):

NSArray *timeZoneNames = [NSTimeZone knownTimeZoneNames];

tzDict contains the abbreviations for daylight saving time for a subset of time zone names. So the algorithm would be to check if we are in DST, then see if tzDict has an entry, and subsitute that or if not, use 

[NSTimeZone abbreviation];

Here are a few other topics on time zones in general.

Daylight saving time and time zone best practices

How can I map tz database names to city and country names?

C# british summer time (BST) timezone abbreviation

http://en.wikipedia.org/wiki/List_of_tz_database_time_zones

GMT timezone conversion in objective c

Community
  • 1
  • 1
Nick T
  • 897
  • 8
  • 30
0

I spent a day on this and wanted to make sure so others don't get hung up on it.

For a UK region/locale, the 'z' field pattern in DateFormatter returns the proper string for the "America/London" timezone (e.g. "GMT" or "BST"). However, if you use the US region/locale, 'z' will give you "GMT+1".

The gotcha happens when an abbreviation for the given target timezone with a given locale doesn't exist. I was using the 'v' field pattern, and with the US region, "Europe/London" was falling back to "United Kingdom Time", which blew up my text label. 'z' is better, it falls back to "GMT+1" while in DST, but still not the desired "BST".

The abbreviationDictionary doesn't consider daylight savings time. It's meant to map an abbreviation to a timezone. You'll find "BST":"Americal/London", but this only applies while observing daylight savings time as mentioned by @Nick T. During standard time, Brits are used to seeing "GMT".

Unfortunately, without recreating timezone functionality yourself, you're left with compromises. I think iOS can do better here.

Reference

Barbituate
  • 363
  • 2
  • 6
  • Actually, using `DateFormatter` for this is kind of silly. You should use https://developer.apple.com/documentation/foundation/timezone/2292835-localizedname on `TimeZone.current`. – Sulthan Apr 24 '20 at 20:06