DST time zone offsets wrong pre-2006 on MingwX64

[jeffdgr8]

Running the following code:

logTzDst(TimeZone.of("America/Los_Angeles"))
logTzDst(TimeZone.of("America/Denver"))
logTzDst(TimeZone.of("America/Chicago"))
logTzDst(TimeZone.of("America/New_York"))

private fun logTzDst(timeZone: TimeZone) {
    println(timeZone)
    var last: String? = null
    for (y in 2024 downTo 1900) {
        val date = LocalDate.parse("$y-08-01")
            .atTime(0, 0)
            .toInstant(timeZone)
        val offset = date.toString().substring(11)
        if (offset != last) {
            println(date)
            println("...")
        }
        last = offset
    }
    println()
}

MingwX64 0.6.0 doesn't adjust for daylight savings time before 2006:

America/Los_Angeles
2024-08-01T07:00:00Z
...
2005-08-01T08:00:00Z
...

America/Denver
2024-08-01T06:00:00Z
...
2005-08-01T07:00:00Z
...

America/Chicago
2024-08-01T05:00:00Z
...
2005-08-01T06:00:00Z
...

America/New_York
2024-08-01T04:00:00Z
...
2005-08-01T05:00:00Z
...

MingwX64 0.5.0 naively adjusts for daylight savings time for all years, which is mostly correct for the previous century, but not for some of those years and pre-1918:

America/Los_Angeles
2024-08-01T07:00:00Z
...

America/Denver
2024-08-01T06:00:00Z
...

America/Chicago
2024-08-01T05:00:00Z
...

America/New_York
2024-08-01T04:00:00Z
...

JVM/iOS/macOS/Linux properly adjust for daylight savings time in the correct years:

America/Los_Angeles
2024-08-01T07:00:00Z
...
1949-08-01T08:00:00Z
...
1948-08-01T07:00:00Z
...
1947-08-01T08:00:00Z
...
1945-08-01T07:00:00Z
...
1941-08-01T08:00:00Z
...
1919-08-01T07:00:00Z
...
1917-08-01T08:00:00Z
...

America/Denver
2024-08-01T06:00:00Z
...
1964-08-01T07:00:00Z
...
1945-08-01T06:00:00Z
...
1941-08-01T07:00:00Z
...
1920-08-01T06:00:00Z
...
1917-08-01T07:00:00Z
...

America/Chicago
2024-08-01T05:00:00Z
...
1917-08-01T06:00:00Z
...

America/New_York
2024-08-01T04:00:00Z
...
1917-08-01T05:00:00Z
...

[dkhalanskyjb]

We use an approach similar to https://learn.microsoft.com/en-us/windows/win32/api/timezoneapi/ns-timezoneapi-dynamic_time_zone_information in our implementation (to my knowledge, it's the only TimeZone API available to us on mingwX64), which queries the Windows registry to obtain timezone information. When I run regedit.exe and open Computer/HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Windows NT/CurrentVersion/Time Zones/Pacific Standard Time/Dynamic DST, I see just two years of historical data: 2006 and 2007. Anything before that is unspecified.

Linux and MacOS typically provide much more complete information in their timezone databases based on https://data.iana.org/, which is why you see accurate results there.

Short of giving our users an option to use a timezone database other than the system one (#201), I don't think we can do anything about this issue. Just ignoring the Windows registry and always supplying our own version of timezone rules is also questionable, as in our research, we've seen Windows users changing the registry values and relying on those changes.