That's because it defaults to zero. We should add the following to:

https://cs.chromium.org/chromium/src/chrome/browser/push_messaging/push_messaging_notification_manager.cc?q=push_messaging_notification_manager&sq=package:chromium&dr=C&l=69

notification_data.timestamp = base::Time::Now();


I'm heading to lunch now - happy to write the fix or review yours if you beat me to it.

Thank you! I wrote the CL https://crrev.com/c/683914 (just pasted your fix :)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f1b84062d58fd80832fccfb828b2d5c25b6b7388

commit f1b84062d58fd80832fccfb828b2d5c25b6b7388
Author: Tetsui Ohkubo <tetsui@chromium.org>
Date: Tue Sep 26 06:08:16 2017

Set timestamp to push messaging default notification.

This CL sets current timestamp to the default notification of push
messaging.
Previously, invalid time was shown when new style notification
chrome://flags/#enable-message-center-new-style-notification
is Enabled (which is ToT default).

TEST=manual
BUG= 768695 

Change-Id: Id253c990d39a8f8df71472c47137b8252a9186d4
Reviewed-on: https://chromium-review.googlesource.com/683914
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504296}
[modify] https://crrev.com/f1b84062d58fd80832fccfb828b2d5c25b6b7388/chrome/browser/push_messaging/push_messaging_notification_manager.cc