Created
May 7, 2019 21:06
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Swift Playground showing unexpected output for historical time zone rules (using example of 8 July 1842 in Europe/Rome)
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| /** | |
| 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) | |
| } |
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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.