static let calendar with autoupdatingCurrent time zone is not working

Question

In my application I have a static gregorian property in SharedCalendar class that is defined like this:

static let gregorian: Calendar = {
    var calendar = Foundation.Calendar(identifier: .gregorian)
    calendar.timeZone = TimeZone.autoupdatingCurrent
    return calendar
}()

When I want to access a day of some date in specific time zone I am calling:

SharedCalendar.gregorian.dateComponents([ .day ], from: someDate).day!

Let's say someDate is Date(timeIntervalSinceReferenceDate: 512658000.0) which is 2017-03-31 13:00:00 +0000.

When I start the app in Vancouver time zone the SharedCalendar.gregorian.timeZone property has value America/Vancouver (autoupdatingCurrent) and the result of SharedCalendar.gregorian.dateComponents([ .day ], from: someDate).day! is 31 which is correct.

When I put the application to background and switch the time zone to Sydney and run the app again the SharedCalendar.gregorian.timeZone property has value Australia/Sydney (autoupdatingCurrent) (which is correct), but the result of SharedCalendar.gregorian.dateComponents([ .day ], from: someDate).day! is 31 which is wrong (should be 1).

When I change the definition of gregorian property to be a var:

var gregorian: Calendar {
    var calendar = Foundation.Calendar(identifier: .gregorian)
    calendar.timeZone = TimeZone.autoupdatingCurrent
    return calendar
}

Everything works properly, which is for America/Vancouver (autoupdatingCurrent) I get 31, and for Australia/Sydney (autoupdatingCurrent) I get 1.

Right now I don't quite understand how TimeZone.autoupdatingCurrent is working. When device's time zone changes the SharedCalendar.gregorian.timeZone reflects the device's time zone, but it looks like SharedCalendar.gregorian is somehow using the old time zone.

Does anyone have explanation of this behaviour?

Answer 1

I reported radar regarding this issue and today Apple responded:

The reason that your static let calendar’s time zone doesn’t update is that you need to issue a call to NSTimeZone.resetSystemTimeZone() to sync up with the system time zone. See the documentation for NSTimeZone.resetSystemTimeZone() for more info: https://developer.apple.com/documentation/foundation/nstimezone/1387189-resetsystemtimezone?language=objc

The reason your var calendar works is because every call to the calendar property actually creates a new computed calendar, which happens to be set to a new time zone representing the current system time zone.

This makes sense, because static let caches system's time zone, and from documentation for NSTimeZone.resetSystemTimeZone we can read:

If the application has cached the system time zone, this method clears that cached object. If you subsequently invoke systemTimeZone, NSTimeZone will attempt to redetermine the system time zone and a new object will be created and cached (see systemTimeZone).