Use {To/From}JavaTime for policy timestamps

components/policy/proto/device_management_local.proto

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package enterprise_management;
 
 import "device_management_backend.proto";
 
 // Wrapper around CloudPolicyResponse/DevicePolicyResponse for caching on disk.
 message CachedCloudPolicyResponse {
   // The DevicePolicyResponse wrapped by this message.
   optional DevicePolicyResponse device_policy = 1;
   // Timestamp noting when the |unmanaged| flag was set. The meaning is
-  // milliseconds since 1970. When caching (deprecated) DevicePolicyResponses,
-  // this timestamp also notes when the response was cached.
+  // milliseconds since Epoch in UTC timezone (Java time). When caching
+  // (deprecated) DevicePolicyResponses, this timestamp also notes when the
+  // response was cached.
   optional uint64 timestamp = 2;
   // Flag that is set to true if this device is not managed.
   optional bool unmanaged = 3;
   // The PolicyFetchResponse wrapped by this message.
   optional PolicyFetchResponse cloud_policy = 4;
 }
 
 // Encapsulates a device ID and the associated device token.
 message DeviceCredentials {
   optional string device_id = 1;
   optional string device_token = 2;
 }