DateFormatter.string still applies Daylight Saving Time to mexican timezones, even when Mexico stoped using daylight saving time this year (2023)

[jasaldivara]

Mexico stopped using daylight saving time

But when I try to format a Date using Foundation's DateFormatter with a mexican TimeZone on Swift, it shows the date as if the daylight saving time were still in use.

Time zones from central Mexico (like "America/Monterrey" and "America/Mexico") are six hours before Universal Time (UTC-6:00), but with Foundation they are currently being formatted as if they were UTC-5:00, which is not correct.

Code example:

import Foundation

let now = Date()
let dtFormatter = DateFormatter()
dtFormatter.timeStyle = .medium
dtFormatter.dateStyle = .none
dtFormatter.timeZone = TimeZone(identifier: "America/Monterrey")
print (dtFormatter.string(from: now))

Result: It prints 9:13:22 PM when the correct local time is 8:13 PM

There is something strange. If I run:

print (dtFormatter.timeZone!.isDaylightSavingTime(for: now))

It prints false, which is correct. But trying to format a Date as string still behaves as if daylight saving time were active.

If I run:

print (TimeZone.timeZoneDataVersion)

It prints 2019c, which make me think that the time zone data is too old

Operating System: Linux, Fedora 38. I'm using the swift-lang package from the Fedora repos

[compnerd]

This information is queried through ICU. The version of the ICU data that is being built and linked against is https://github.com/apple/swift/blob/main/utils/update_checkout/update-checkout-config.json#L120 (65.1), which likely does not have the updated CLDR data.

[jasaldivara]

Is there a reason why Swift depends on ICU version 65 instead of it's latest release?

[fred-sch]

We also observe the described behavior. Here is a small test case that will find time zone identifiers which show the same behavior. Maybe this is of any use

import Foundation
import XCTest

extension DateFormatter {
    public convenience init(format: String, timeZone: TimeZone = TimeZone(abbreviation: "UTC")!, locale: Locale = .current) {
        self.init()
        self.dateFormat = format
        self.timeZone = timeZone
        self.locale = locale
    }
}

final class TimeZoneTests: XCTestCase {
    func testTimeZonesFromUTCOffset() {

        let now = Date(timeIntervalSince1970: Date().timeIntervalSince1970.rounded())

        for identifier in TimeZone.knownTimeZoneIdentifiers {
            guard let timeZoneFromIdentifier = TimeZone(identifier: identifier) else {
                XCTFail("Cannot initialize timezone from identifier \(identifier)")
                continue
            }

            let gmtOffset = timeZoneFromIdentifier.secondsFromGMT()
            let timeZoneFromGMTOffset = TimeZone(secondsFromGMT: gmtOffset)!

            let formatterFromIdentifier = DateFormatter(format: "yyyyMMdd'T'HH:mm:ss", timeZone: timeZoneFromIdentifier)
            let formatterFromGMTOffset = DateFormatter(format: "yyyyMMdd'T'HH:mm:ss", timeZone: timeZoneFromGMTOffset)

            let formatted = formatterFromIdentifier.string(from: now)

            guard let nowAfterRoundtrip = formatterFromGMTOffset.date(from: formatted) else {
                XCTFail("Could not parse date \(formatted) with \(formatterFromGMTOffset.description)")
                continue
            }

            XCTAssertEqual(now, nowAfterRoundtrip, "Roundtrip did not work for timezone \(identifier)")
        }
    }
}