Created
May 7, 2019 21:06
-
-
Save stephent/5121334efd175f6772327fc352732530 to your computer and use it in GitHub Desktop.
Swift Playground showing unexpected output for historical time zone rules (using example of 8 July 1842 in Europe/Rome)
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
| /** | |
| Playground illustrating incorrect result for secondsFromGMT for 8 July 1842 in time zone Europe/Rome. A current date is included to illustrate that secondsFromGMT and the output of the date formatter are in agreement. | |
| For 8 July 1842, expected output is: | |
| Describing date: 1842-07-09 05:39:33 +0000 | |
| Seconds from GMT: 2996 | |
| Abbreviation: LMT | |
| Formatted string: 6:29:29 AM Jul 09, 1842, GMT+00:49:56 | |
| The abbreviation LMT comes from line 1660 of the current IANA tz database, filename "europe" | |
| */ | |
| import UIKit | |
| let iso8601df = ISO8601DateFormatter() | |
| iso8601df.timeZone = TimeZone(identifier: "Etc/UTC") | |
| let europeRome = TimeZone(identifier: "Europe/Rome") | |
| let today = Date() | |
| // Formatter to display time of day | |
| let tf = DateFormatter() | |
| tf.timeStyle = .medium | |
| // Formatter to display date | |
| let df = DateFormatter() | |
| df.setLocalizedDateFormatFromTemplate("MMM dd, yyyy, ZZZZ") | |
| func describe(date:Date, using timeZone:TimeZone) { | |
| print("***") | |
| print("Describing date: \(date)") | |
| print("Seconds from GMT: \(String(describing: timeZone.secondsFromGMT(for: date)))") | |
| print("Abbreviation: \(timeZone.abbreviation(for: date) ?? "?")") | |
| print("Formatted string: \(tf.string(from: date)) \(df.string(from: date))") | |
| print("***") | |
| } | |
| if let tz = europeRome, let longLongAgo = iso8601df.date(from: "1842-07-09T05:39:33+00:00") { | |
| tf.timeZone = tz | |
| df.timeZone = tz | |
| describe(date: today, using: tz) | |
| describe(date: longLongAgo, using: tz) | |
| } |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
When run, the playground produces the following debug output:
`
Describing date: 2019-05-07 20:56:07 +0000
Seconds from GMT: 7200
Abbreviation: GMT+2
Formatted string: 10:56:07 PM May 07, 2019, GMT+02:00
Describing date: 1842-07-09 05:39:33 +0000
Seconds from GMT: 3600
Abbreviation: GMT+1
Formatted string: 6:29:29 AM Jul 09, 1842, GMT+00:49:56
`
See comments in Gist for expected output for the 1842 date.