diff --git a/java/util/src/main/java/com/google/protobuf/util/Durations.java b/java/util/src/main/java/com/google/protobuf/util/Durations.java
index bbcac19b05fa9..1dd153229a690 100644
--- a/java/util/src/main/java/com/google/protobuf/util/Durations.java
+++ b/java/util/src/main/java/com/google/protobuf/util/Durations.java
@@ -20,6 +20,7 @@
 import static com.google.protobuf.util.Timestamps.NANOS_PER_SECOND;
 
 import com.google.common.annotations.GwtIncompatible;
+import com.google.common.annotations.J2ktIncompatible;
 import com.google.common.base.Strings;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.errorprone.annotations.CompileTimeConstant;
@@ -144,6 +145,7 @@ public static boolean isPositive(Duration duration) {
    */
   @CanIgnoreReturnValue
   @GwtIncompatible("Depends on String.format which is not supported in Xplat.")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   public static Duration checkNotNegative(Duration duration) {
     checkArgument(!isNegative(duration), "duration (%s) must not be negative", toString(duration));
@@ -158,6 +160,7 @@ public static Duration checkNotNegative(Duration duration) {
    */
   @CanIgnoreReturnValue
   @GwtIncompatible("Depends on String.format which is not supported in Xplat.")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   public static Duration checkPositive(Duration duration) {
     checkArgument(isPositive(duration), "duration (%s) must be positive", toString(duration));
@@ -201,6 +204,7 @@ public static Duration checkValid(Duration.Builder durationBuilder) {
    * @throws IllegalArgumentException if the given duration is not in the valid range.
    */
   @GwtIncompatible("Depends on String.format which is not supported in Xplat.")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   public static String toString(Duration duration) {
     checkValid(duration);
@@ -230,6 +234,7 @@ public static String toString(Duration duration) {
    * @throws ParseException if the string is not in the duration format
    */
   @GwtIncompatible("ParseException is not supported in Xplat")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   public static Duration parse(String value) throws ParseException {
     // Must end with "s".
@@ -281,6 +286,7 @@ public static Duration parse(String value) throws ParseException {
    * @throws IllegalArgumentException if parsing fails
    */
   @GwtIncompatible("ParseException is not supported in Xplat")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   public static Duration parseUnchecked(@CompileTimeConstant String value) {
     try {
diff --git a/java/util/src/main/java/com/google/protobuf/util/Timestamps.java b/java/util/src/main/java/com/google/protobuf/util/Timestamps.java
index 8d60a93e93a1e..40146e942e6a8 100644
--- a/java/util/src/main/java/com/google/protobuf/util/Timestamps.java
+++ b/java/util/src/main/java/com/google/protobuf/util/Timestamps.java
@@ -14,6 +14,7 @@
 import static com.google.common.math.LongMath.checkedSubtract;
 
 import com.google.common.annotations.GwtIncompatible;
+import com.google.common.annotations.J2ktIncompatible;
 import com.google.common.base.Strings;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.errorprone.annotations.CompileTimeConstant;
@@ -66,6 +67,7 @@ public final class Timestamps {
 
   @GwtIncompatible("Date formatting is not supported in non JVM java.time")
   @J2ObjCIncompatible
+  @J2ktIncompatible
   private static final ThreadLocal<SimpleDateFormat> timestampFormat =
       new ThreadLocal<SimpleDateFormat>() {
         @Override
@@ -76,6 +78,7 @@ protected SimpleDateFormat initialValue() {
 
   @GwtIncompatible("Date formatting is not supported in non JVM java.time")
   @J2ObjCIncompatible
+  @J2ktIncompatible
   private static SimpleDateFormat createTimestampFormat() {
     SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss", Locale.ENGLISH);
     GregorianCalendar calendar = new GregorianCalendar(TimeZone.getTimeZone("UTC"));
@@ -200,6 +203,7 @@ public static Timestamp checkValid(Timestamp.Builder timestampBuilder) {
    * @throws IllegalArgumentException if the given timestamp is not in the valid range.
    */
   @GwtIncompatible("Depends on String.format which is not supported in Xplat.")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   public static String toString(Timestamp timestamp) {
     checkValid(timestamp);
@@ -231,6 +235,7 @@ public static String toString(Timestamp timestamp) {
    * @throws ParseException if parsing fails
    */
   @GwtIncompatible("ParseException is not supported in Xplat")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   public static Timestamp parse(String value) throws ParseException {
     int dayOffset = value.indexOf('T');
@@ -298,6 +303,7 @@ public static Timestamp parse(String value) throws ParseException {
    * @throws IllegalArgumentException if parsing fails
    */
   @GwtIncompatible("Depends on String.format which is not supported in Xplat.")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   public static Timestamp parseUnchecked(@CompileTimeConstant String value) {
     try {
@@ -312,21 +318,25 @@ public static Timestamp parseUnchecked(@CompileTimeConstant String value) {
   // the following 3 constants contain references to java.time.Instant methods (if that class is
   // available at runtime); otherwise, they are null.
   @GwtIncompatible("Uses reflection to access methods of java.time.Instant")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   @Nullable
   private static final Method INSTANT_NOW = instantMethod("now");
 
   @GwtIncompatible("Uses reflection to access methods of java.time.Instant")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   @Nullable
   private static final Method INSTANT_GET_EPOCH_SECOND = instantMethod("getEpochSecond");
 
   @GwtIncompatible("Uses reflection to access methods of java.time.Instant")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   @Nullable
   private static final Method INSTANT_GET_NANO = instantMethod("getNano");
 
   @GwtIncompatible("Uses reflection to access methods of java.time.Instant")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   @Nullable
   private static Method instantMethod(String methodName) {
@@ -345,6 +355,7 @@ private static Method instantMethod(String methodName) {
    * instance to read the current time.
    */
   @GwtIncompatible("Uses reflection to access methods of java.time.Instant")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   @SuppressWarnings("nullness")
   public static Timestamp now() {
@@ -515,6 +526,7 @@ static Timestamp normalizedTimestamp(long seconds, int nanos) {
   }
 
   @GwtIncompatible("Depends on String.format which is not supported in Xplat.")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   private static long parseTimezoneOffset(String value) throws ParseException {
     int pos = value.indexOf(':');
@@ -533,6 +545,7 @@ private static long parseTimezoneOffset(String value) throws ParseException {
   }
 
   @GwtIncompatible("Depends on String.format which is not supported in Xplat.")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   static int parseNanos(String value) throws ParseException {
     int result = 0;
@@ -550,6 +563,7 @@ static int parseNanos(String value) throws ParseException {
 
   /** Format the nano part of a timestamp or a duration. */
   @GwtIncompatible("Depends on String.format which is not supported in Xplat.")
+  @J2ktIncompatible
   @J2ObjCIncompatible
   static String formatNanos(int nanos) {
     // Determine whether to use 3, 6, or 9 digits for the nano part.