Return FixedOffsetTimeZone when DST is disabled on Windows (#583)

Align timezone behavior with Java by returning a FixedOffsetTimeZone
instead of a RegionTimeZone when DynamicDaylightTimeDisabled is set.

Previously, Windows users with DST disabled would receive a 
RegionTimeZone with a confusing name. Now they receive a properly-typed
FixedOffsetTimeZone with a clear GMT-prefixed name (e.g., GMT+01:00).

Changes:
- Extract asTimeZone(prefix) utility for consistent timezone creation
- Refactor timezone ID parsing to use new utility
- Extract currentSystemDefaultFromDtzi helper for testability
- Add comprehensive tests for both DST enabled/disabled scenarios

Fixes #575

Author: DmitryNekrasov

core/commonKotlin/src/TimeZone.kt

@@ -54,17 +54,11 @@ public actual open class TimeZone internal constructor() {
                 ) {
                     val prefix = zoneId.take(3)
                     val offset = lenientOffsetFormat.parse(zoneId.substring(3))
-                    return when (offset.totalSeconds) {
-                        0 -> FixedOffsetTimeZone(offset, prefix)
-                        else -> FixedOffsetTimeZone(offset, "$prefix$offset")
-                    }
+                    return offset.asTimeZone(prefix)
                 }
                 if (zoneId.startsWith("UT+") || zoneId.startsWith("UT-")) {
                     val offset = lenientOffsetFormat.parse(zoneId.substring(2))
-                    return when (offset.totalSeconds) {
-                        0 -> FixedOffsetTimeZone(offset, "UT")
-                        else -> FixedOffsetTimeZone(offset, "UT$offset")
-                    }
+                    return offset.asTimeZone("UT")
                 }
             } catch (e: DateTimeFormatException) {
                 throw IllegalTimeZoneException(e)

core/commonKotlin/src/UtcOffset.kt

@@ -108,3 +108,9 @@ public actual fun UtcOffset(hours: Int? = null, minutes: Int? = null, seconds: I
             UtcOffset.ofSeconds(seconds ?: 0)
         }
     }
+
+internal fun UtcOffset.asTimeZone(prefix: String): FixedOffsetTimeZone =
+    when (totalSeconds) {
+        0 -> FixedOffsetTimeZone(this, prefix)
+        else -> FixedOffsetTimeZone(this, "$prefix$this")
+    }

core/windows/src/internal/TzdbInRegistry.kt

@@ -90,18 +90,19 @@ internal class TzdbInRegistry: TimeZoneDatabase {
         val dtzi = alloc<DYNAMIC_TIME_ZONE_INFORMATION>()
         val result = GetDynamicTimeZoneInformation(dtzi.ptr)
         check(result != TIME_ZONE_ID_INVALID) { "The current system time zone is invalid: ${getLastWindowsError()}" }
+        return currentSystemDefaultFromDtzi(dtzi)
+    }
+
+    internal fun currentSystemDefaultFromDtzi(dtzi: DYNAMIC_TIME_ZONE_INFORMATION): Pair<String, TimeZone> {
         val windowsName = dtzi.TimeZoneKeyName.toKStringFromUtf16()
         val ianaTzName = if (windowsName == "Coordinated Universal Time") "UTC" else windowsToStandard[windowsName]
                 ?: throw IllegalStateException("Unknown time zone name '$windowsName'")
         val tz = windowsToRules[windowsName]
         check(tz != null) { "The system time zone is set to a value rules for which are not known: '$windowsName'" }
-        val rules = if (dtzi.DynamicDaylightTimeDisabled == 0.convert<BOOLEAN>()) {
-            tz
-        } else {
-            // the user explicitly disabled DST transitions, so
-            TimeZoneRulesCommon(UtcOffset(minutes = -(dtzi.Bias + dtzi.StandardBias)), RecurringZoneRules(emptyList()))
-        }
-        return ianaTzName to RegionTimeZone(rules, ianaTzName)
+        return ianaTzName to if (dtzi.DynamicDaylightTimeDisabled == 0.convert<BOOLEAN>())
+            RegionTimeZone(tz, ianaTzName)
+        else  // the user explicitly disabled DST transitions, so
+            UtcOffset(minutes = -(dtzi.Bias + dtzi.StandardBias)).asTimeZone("GMT")
     }
 }
 

core/windows/test/TimeZoneRulesCompleteTest.kt

@@ -19,6 +19,26 @@ import kotlin.time.Clock
 
 class TimeZoneRulesCompleteTest {
 
+    @Test
+    fun dynamicDaylightTimeDisabled() {
+        memScoped {
+            val dtzi = alloc<DYNAMIC_TIME_ZONE_INFORMATION>()
+            val result = GetDynamicTimeZoneInformation(dtzi.ptr)
+            check(result != TIME_ZONE_ID_INVALID) { "The current system time zone is invalid: ${getLastWindowsError()}" }
+
+            val tzdb = TzdbInRegistry()
+
+            dtzi.DynamicDaylightTimeDisabled = 0u
+            val (_, tzWithDst) = tzdb.currentSystemDefaultFromDtzi(dtzi)
+            assertTrue(tzWithDst is RegionTimeZone, "Expected RegionTimeZone, got ${tzWithDst::class}")
+
+            dtzi.DynamicDaylightTimeDisabled = 1u
+            val (_, tzWithoutDst) = tzdb.currentSystemDefaultFromDtzi(dtzi)
+            assertTrue(tzWithoutDst is FixedOffsetTimeZone, "Expected FixedOffsetTimeZone, got ${tzWithoutDst::class}")
+            assertTrue(tzWithoutDst.toString().startsWith("GMT"), "Expected GMT timezone, got $tzWithoutDst")
+        }
+    }
+
     /** Tests that all transitions that our system recognizes are actually there. */
     @OptIn(ExperimentalStdlibApi::class)
     @Test