Make updated JARs reproducible across time zones

Issue: #3690

gradle/plugins/common/src/main/kotlin/junitbuild/java/UpdateJarAction.kt

@@ -8,18 +8,31 @@ import org.gradle.api.provider.Property
 import org.gradle.jvm.toolchain.JavaLauncher
 import org.gradle.process.ExecOperations
 import java.time.Instant
+import java.time.LocalDateTime
+import java.time.ZoneId
+import java.time.ZoneOffset
 import javax.inject.Inject
 
 abstract class UpdateJarAction @Inject constructor(private val operations: ExecOperations): Action<Task> {
 
+    companion object {
+        // Since ZipCopyAction.CONSTANT_TIME_FOR_ZIP_ENTRIES is in the default time zone (see its Javadoc),
+        // we're converting it to the same time in UTC here to make the jar reproducible regardless of the
+        // build's time zone.
+        private val CONSTANT_TIME_FOR_ZIP_ENTRIES = LocalDateTime.ofInstant(Instant.ofEpochMilli(ZipCopyAction.CONSTANT_TIME_FOR_ZIP_ENTRIES), ZoneId.systemDefault())
+            .toInstant(ZoneOffset.UTC)
+            .toString()
+    }
+
     abstract val javaLauncher: Property<JavaLauncher>
 
     abstract val args: ListProperty<String>
 
     init {
         args.convention(listOf(
             "--update",
-            "--date=${Instant.ofEpochMilli(ZipCopyAction.CONSTANT_TIME_FOR_ZIP_ENTRIES)}",
+            // Use a constant time to make the JAR reproducible.
+            "--date=$CONSTANT_TIME_FOR_ZIP_ENTRIES",
         ))
     }