#485: Add tests and fallback logic for retrieving available time zone IDs on Darwin

Author: DmitryNekrasov

core/darwin/src/internal/TimeZoneNative.kt

@@ -15,7 +15,11 @@ internal actual fun timeZoneById(zoneId: String): TimeZone =
     RegionTimeZone(tzdb.getOrThrow().rulesForId(zoneId), zoneId)
 
 internal actual fun getAvailableZoneIds(): Set<String> =
-    tzdb.getOrThrow().availableTimeZoneIds()
+    runCatching { tzdb.getOrThrow().availableTimeZoneIds() }
+        .getOrElse { getAvailableZoneIdsFoundation() }
+
+internal fun getAvailableZoneIdsFoundation(): Set<String> =
+    NSTimeZone.knownTimeZoneNames.mapTo(mutableSetOf()) { it.toString() }
 
 private val tzdb = runCatching { TzdbOnFilesystem(Path.fromString(defaultTzdbPath())) }
 

core/darwin/test/TimeZoneNativeTest.kt

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2019-2025 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+package kotlinx.datetime.test
+
+import kotlinx.datetime.internal.getAvailableZoneIds
+import kotlinx.datetime.internal.getAvailableZoneIdsFoundation
+import kotlin.test.Test
+import kotlin.test.assertTrue
+
+class TimeZoneNativeTest {
+
+    @Test
+    fun getAvailableZoneIdsReturnsValidTimezoneSet() {
+        assertReturnsNonEmptySetOfTimezoneStrings(getAvailableZoneIds())
+    }
+
+    @Test
+    fun getAvailableZoneIdsFoundationReturnsValidTimezoneSet() {
+        assertReturnsNonEmptySetOfTimezoneStrings(getAvailableZoneIdsFoundation())
+    }
+
+    @Test
+    fun getAvailableZoneIdsContainsExpectedTimezoneIDs() {
+        assertAvailableZoneIdsContainsExpectedTimezoneIDs(getAvailableZoneIds())
+    }
+
+    @Test
+    fun getAvailableZoneIdsFoundationContainsExpectedTimezoneIDs() {
+        assertAvailableZoneIdsContainsExpectedTimezoneIDs(getAvailableZoneIdsFoundation())
+    }
+
+    private fun assertReturnsNonEmptySetOfTimezoneStrings(zoneIds: Set<String>) {
+        assertTrue(zoneIds.isNotEmpty(), "Zone IDs should not be empty")
+        assertTrue(zoneIds.all { it.isNotBlank() }, "All zone IDs should be non-blank")
+        assertTrue("UTC" in zoneIds || "GMT" in zoneIds, "Should contain UTC or GMT")
+        assertTrue(zoneIds.any { it.contains("America") }, "Should contain America timezones")
+        assertTrue(zoneIds.any { it.contains("Europe") }, "Should contain Europe timezones")
+    }
+
+    private fun assertAvailableZoneIdsContainsExpectedTimezoneIDs(zoneIds: Set<String>) {
+        val expectedZones = listOf("GMT", "America/New_York", "Europe/London", "Asia/Tokyo", "Australia/Sydney")
+        assertTrue(expectedZones.all { it in zoneIds }, "Should contain all common timezone")
+    }
+}