Deprecate TimeZone serialization

core/common/src/TimeZone.kt

@@ -33,7 +33,6 @@ import kotlin.time.Instant
  *
  * @sample kotlinx.datetime.test.samples.TimeZoneSamples.usage
  */
-@Serializable(with = TimeZoneSerializer::class)
 public expect open class TimeZone {
     /**
      * Returns the identifier string of the time zone.
@@ -163,6 +162,15 @@ public expect open class TimeZone {
          * @sample kotlinx.datetime.test.samples.TimeZoneSamples.availableZoneIds
          */
         public val availableZoneIds: Set<String>
+
+        /** @suppress */
+        @Deprecated(
+            "Serializing TimeZone is discouraged, " +
+                    "as deserialization can fail depending on the configuration. " +
+                    "Please serialize the string id instead.",
+            level = DeprecationLevel.WARNING,
+        )
+        public fun serializer(): kotlinx.serialization.KSerializer<TimeZone>
     }
 
     /**
@@ -235,7 +243,6 @@ public expect open class TimeZone {
  *
  * @sample kotlinx.datetime.test.samples.TimeZoneSamples.FixedOffsetTimeZoneSamples.casting
  */
-@Serializable(with = FixedOffsetTimeZoneSerializer::class)
 public expect class FixedOffsetTimeZone : TimeZone {
     /**
      * Constructs a time zone with the fixed [offset] from UTC.
@@ -253,6 +260,18 @@ public expect class FixedOffsetTimeZone : TimeZone {
 
     @Deprecated("Use offset.totalSeconds", ReplaceWith("offset.totalSeconds"))
     public val totalSeconds: Int
+
+    /** @suppress */
+    public companion object {
+        /** @suppress */
+        @Deprecated(
+            "Serializing FixedOffsetTimeZone is discouraged, " +
+                    "as deserialization can fail or return a non-fixed-offset zone depending on the configuration. " +
+                    "Please serialize the string id instead.",
+            level = DeprecationLevel.WARNING,
+        )
+        public fun serializer(): kotlinx.serialization.KSerializer<FixedOffsetTimeZone>
+    }
 }
 
 @Deprecated("Use FixedOffsetTimeZone or UtcOffset instead", ReplaceWith("FixedOffsetTimeZone"))

core/common/src/serializers/TimeZoneSerializers.kt

@@ -1,12 +1,11 @@
 /*
- * Copyright 2019-2021 JetBrains s.r.o.
+ * Copyright 2025 JetBrains s.r.o.
  * 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.serializers
 
 import kotlinx.datetime.*
-import kotlinx.datetime.format.DateTimeFormat
 import kotlinx.serialization.*
 import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
@@ -16,6 +15,10 @@ import kotlinx.serialization.encoding.*
  *
  * JSON example: `"Europe/Berlin"`
  */
+@Deprecated(
+    "Serializing TimeZone is discouraged. Please serialize the string id instead.",
+    level = DeprecationLevel.WARNING,
+)
 public object TimeZoneSerializer: KSerializer<TimeZone> {
 
     override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("kotlinx.datetime.TimeZone", PrimitiveKind.STRING)
@@ -33,6 +36,10 @@ public object TimeZoneSerializer: KSerializer<TimeZone> {
  *
  * JSON example: `"+02:00"`
  */
+@Deprecated(
+    "Serializing FixedOffsetTimeZoneSerializer is discouraged. Please serialize the string id instead.",
+    level = DeprecationLevel.WARNING,
+)
 public object FixedOffsetTimeZoneSerializer: KSerializer<FixedOffsetTimeZone> {
 
     override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("kotlinx.datetime.FixedOffsetTimeZone", PrimitiveKind.STRING)
@@ -51,60 +58,3 @@ public object FixedOffsetTimeZoneSerializer: KSerializer<FixedOffsetTimeZone> {
     }
 
 }
-
-/**
- * A serializer for [UtcOffset] that uses the extended ISO 8601 representation.
- *
- * JSON example: `"+02:00"`
- *
- * @see UtcOffset.Formats.ISO
- */
-public object UtcOffsetIso8601Serializer : KSerializer<UtcOffset>
-by UtcOffset.Formats.ISO.asKSerializer("kotlinx.datetime.UtcOffset/ISO")
-
-/**
- * A serializer for [UtcOffset] that uses the default [UtcOffset.toString]/[UtcOffset.parse].
- *
- * JSON example: `"+02:00"`
- */
-@Deprecated("Use UtcOffset.serializer() instead", ReplaceWith("UtcOffset.serializer()"))
-public object UtcOffsetSerializer: KSerializer<UtcOffset> {
-
-    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("kotlinx.datetime.UtcOffset", PrimitiveKind.STRING)
-
-    override fun deserialize(decoder: Decoder): UtcOffset {
-        return UtcOffset.parse(decoder.decodeString())
-    }
-
-    override fun serialize(encoder: Encoder, value: UtcOffset) {
-        encoder.encodeString(value.toString())
-    }
-
-}
-
-/**
- * An abstract serializer for [UtcOffset] values that uses
- * a custom [DateTimeFormat] to serialize and deserialize the value.
- *
- * [name] is the name of the serializer.
- * The [SerialDescriptor.serialName] of the resulting serializer is `kotlinx.datetime.UtcOffset/serializer/`[name].
- * [SerialDescriptor.serialName] must be unique across all serializers in the same serialization context.
- * When defining a serializer in a library, it is recommended to use the fully qualified class name in [name]
- * to avoid conflicts with serializers defined by other libraries and client code.
- *
- * This serializer is abstract and must be subclassed to provide a concrete serializer.
- * Example:
- * ```
- * // serializes the UTC offset UtcOffset(hours = 2) as the string "+0200"
- * object FourDigitOffsetSerializer : FormattedUtcOffsetSerializer(
- *     "my.package.FOUR_DIGITS", UtcOffset.Formats.FOUR_DIGITS
- * )
- * ```
- *
- * Note that [UtcOffset] is [kotlinx.serialization.Serializable] by default,
- * so it is not necessary to create custom serializers when the format is not important.
- * Additionally, [UtcOffsetSerializer] is provided for the ISO 8601 format.
- */
-public abstract class FormattedUtcOffsetSerializer(
-    name: String, format: DateTimeFormat<UtcOffset>
-) : KSerializer<UtcOffset> by format.asKSerializer("kotlinx.datetime.UtcOffset/serializer/$name")

core/common/src/serializers/UtcOffsetSerializers.kt

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2019-2021 JetBrains s.r.o.
+ * 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.serializers
+
+import kotlinx.datetime.*
+import kotlinx.datetime.format.DateTimeFormat
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+
+/**
+ * A serializer for [UtcOffset] that uses the extended ISO 8601 representation.
+ *
+ * JSON example: `"+02:00"`
+ *
+ * @see UtcOffset.Formats.ISO
+ */
+public object UtcOffsetIso8601Serializer : KSerializer<UtcOffset>
+by UtcOffset.Formats.ISO.asKSerializer("kotlinx.datetime.UtcOffset/ISO")
+
+/**
+ * A serializer for [UtcOffset] that uses the default [UtcOffset.toString]/[UtcOffset.parse].
+ *
+ * JSON example: `"+02:00"`
+ */
+@Deprecated("Use UtcOffset.serializer() instead", ReplaceWith("UtcOffset.serializer()"))
+public object UtcOffsetSerializer: KSerializer<UtcOffset> {
+
+    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("kotlinx.datetime.UtcOffset", PrimitiveKind.STRING)
+
+    override fun deserialize(decoder: Decoder): UtcOffset {
+        return UtcOffset.parse(decoder.decodeString())
+    }
+
+    override fun serialize(encoder: Encoder, value: UtcOffset) {
+        encoder.encodeString(value.toString())
+    }
+
+}
+
+/**
+ * An abstract serializer for [UtcOffset] values that uses
+ * a custom [DateTimeFormat] to serialize and deserialize the value.
+ *
+ * [name] is the name of the serializer.
+ * The [SerialDescriptor.serialName] of the resulting serializer is `kotlinx.datetime.UtcOffset/serializer/`[name].
+ * [SerialDescriptor.serialName] must be unique across all serializers in the same serialization context.
+ * When defining a serializer in a library, it is recommended to use the fully qualified class name in [name]
+ * to avoid conflicts with serializers defined by other libraries and client code.
+ *
+ * This serializer is abstract and must be subclassed to provide a concrete serializer.
+ * Example:
+ * ```
+ * // serializes the UTC offset UtcOffset(hours = 2) as the string "+0200"
+ * object FourDigitOffsetSerializer : FormattedUtcOffsetSerializer(
+ *     "my.package.FOUR_DIGITS", UtcOffset.Formats.FOUR_DIGITS
+ * )
+ * ```
+ *
+ * Note that [UtcOffset] is [kotlinx.serialization.Serializable] by default,
+ * so it is not necessary to create custom serializers when the format is not important.
+ * Additionally, [UtcOffsetSerializer] is provided for the ISO 8601 format.
+ */
+public abstract class FormattedUtcOffsetSerializer(
+    name: String, format: DateTimeFormat<UtcOffset>
+) : KSerializer<UtcOffset> by format.asKSerializer("kotlinx.datetime.UtcOffset/serializer/$name")

core/commonKotlin/src/TimeZone.kt

@@ -14,7 +14,6 @@ import kotlinx.datetime.serializers.*
 import kotlinx.serialization.Serializable
 import kotlin.time.Instant
 
-@Serializable(with = TimeZoneSerializer::class)
 public actual open class TimeZone internal constructor() {
 
     public actual companion object {
@@ -78,6 +77,15 @@ public actual open class TimeZone internal constructor() {
 
         public actual val availableZoneIds: Set<String>
             get() = getAvailableZoneIds()
+
+        @Deprecated(
+            "Serializing TimeZone is discouraged, " +
+                    "as deserialization can fail depending on the configuration. " +
+                    "Please serialize the string id instead.",
+            level = DeprecationLevel.WARNING,
+        )
+        @Suppress("DEPRECATION")
+        public actual fun serializer(): kotlinx.serialization.KSerializer<TimeZone> = TimeZoneSerializer
     }
 
     public actual open val id: String
@@ -126,7 +134,6 @@ public actual open class TimeZone internal constructor() {
     actual override fun toString(): String = id
 }
 
-@Serializable(with = FixedOffsetTimeZoneSerializer::class)
 public actual class FixedOffsetTimeZone internal constructor(public actual val offset: UtcOffset, override val id: String) : TimeZone() {
 
     public actual constructor(offset: UtcOffset) : this(offset, offset.toString())
@@ -144,6 +151,20 @@ public actual class FixedOffsetTimeZone internal constructor(public actual val o
 
     override fun instantToLocalDateTime(instant: Instant): LocalDateTime = instant.toLocalDateTime(offset)
     override fun localDateTimeToInstant(dateTime: LocalDateTime): Instant = dateTime.toInstant(offset)
+
+    /** @suppress */
+    public actual companion object {
+        /** @suppress */
+        @Deprecated(
+            "Serializing FixedOffsetTimeZone is discouraged, " +
+                    "as deserialization can fail or return a non-fixed-offset zone depending on the configuration. " +
+                    "Please serialize the string id instead.",
+            level = DeprecationLevel.WARNING,
+        )
+        @Suppress("DEPRECATION")
+        public actual fun serializer(): kotlinx.serialization.KSerializer<FixedOffsetTimeZone> =
+            FixedOffsetTimeZoneSerializer
+    }
 }
 
 

core/jvm/src/TimeZoneJvm.kt

@@ -9,15 +9,13 @@
 package kotlinx.datetime
 
 import kotlinx.datetime.serializers.*
-import kotlinx.serialization.Serializable
 import java.time.DateTimeException
 import java.time.ZoneId
 import java.time.ZoneOffset as jtZoneOffset
 import kotlin.time.Instant
 import kotlin.time.toJavaInstant
 import kotlin.time.toKotlinInstant
 
-@Serializable(with = TimeZoneSerializer::class)
 public actual open class TimeZone internal constructor(internal val zoneId: ZoneId) {
     public actual val id: String get() = zoneId.id
 
@@ -71,6 +69,15 @@ public actual open class TimeZone internal constructor(internal val zoneId: Zone
         }
 
         public actual val availableZoneIds: Set<String> get() = ZoneId.getAvailableZoneIds()
+
+        @Deprecated(
+            "Serializing TimeZone is discouraged, " +
+                    "as deserialization can fail depending on the configuration. " +
+                    "Please serialize the string id instead.",
+            level = DeprecationLevel.WARNING,
+        )
+        @Suppress("DEPRECATION")
+        public actual fun serializer(): kotlinx.serialization.KSerializer<TimeZone> = TimeZoneSerializer
     }
 }
 
@@ -83,14 +90,27 @@ private val ZoneId.isFixedOffset: Boolean
         false // Happens for America/Costa_Rica, Africa/Cairo, Egypt
     }
 
-@Serializable(with = FixedOffsetTimeZoneSerializer::class)
 public actual class FixedOffsetTimeZone
 internal constructor(public actual val offset: UtcOffset, zoneId: ZoneId): TimeZone(zoneId) {
 
     public actual constructor(offset: UtcOffset) : this(offset, offset.zoneOffset)
 
     @Deprecated("Use offset.totalSeconds", ReplaceWith("offset.totalSeconds"))
     public actual val totalSeconds: Int get() = offset.totalSeconds
+
+    /** @suppress */
+    public actual companion object {
+        /** @suppress */
+        @Deprecated(
+            "Serializing FixedOffsetTimeZone is discouraged, " +
+                    "as deserialization can fail or return a non-fixed-offset zone depending on the configuration. " +
+                    "Please serialize the string id instead.",
+            level = DeprecationLevel.WARNING,
+        )
+        @Suppress("DEPRECATION")
+        public actual fun serializer(): kotlinx.serialization.KSerializer<FixedOffsetTimeZone> =
+            FixedOffsetTimeZoneSerializer
+    }
 }
 
 public actual fun TimeZone.offsetAt(instant: Instant): UtcOffset =

integration-testing/serialization/common/test/TimeZoneSerializationTest.kt

@@ -34,23 +34,15 @@ class TimeZoneSerializationTest {
         }
     }
 
+    @Suppress("DEPRECATION")
     @Test
     fun testZoneOffsetSerialization() {
         zoneOffsetSerialization(FixedOffsetTimeZoneSerializer)
     }
 
+    @Suppress("DEPRECATION")
     @Test
     fun testSerialization() {
         serialization(TimeZoneSerializer)
     }
-
-    @Test
-    fun testDefaultSerializers() {
-        assertKSerializerName<FixedOffsetTimeZone>(
-            "kotlinx.datetime.FixedOffsetTimeZone", Json.serializersModule.serializer()
-        )
-        zoneOffsetSerialization(Json.serializersModule.serializer())
-        assertKSerializerName<TimeZone>("kotlinx.datetime.TimeZone", Json.serializersModule.serializer())
-        serialization(Json.serializersModule.serializer())
-    }
 }