Ensure post_publish_timestamp in AMP_Post_Template is UTC for human_time_diff()

[westonruter]

Summary

As reported in a support forum topic, the relative publish time displayed on the legacy post template is incorrect when the timezone is not UTC.

For example, on my site I have the timezone set to America/Los_Angeles. Nevertheless, when I publish my post and I immediately view the legacy AMP post template, I see "7 hours ago". This is because my timezone is UTC-7. The problem is that the post_publish_timestamp was being populated with get_the_date( 'U', $this->post ) which returns a non-GMT “Unix timestamp” (which AFAIK doesn't even make sense, since this value should be timezone-independent). Nevertheless, since get_the_date( 'U', $this->post ) is getting the blog's timezone offset, it then fails to render the right result in the template:

amp-wp/templates/meta-time.php

Lines 25 to 31 in 5191d35

	echo esc_html(
	sprintf(
	/* translators: %s: the human-readable time difference. */
	__( '%s ago', 'amp' ),
	human_time_diff( $this->get( 'post_publish_timestamp' ), time() )
	)
	);

Here time() is the Unix timestamp of seconds since January 1, 1970 00:00:00 UTC. But $post_publish_timestamp is actually 7 hours before that on my install.

The fix is to not call get_the_date() directly, but to call the underlying get_post_time() which does allow you to specifically request the GMT time.

Since previously get_the_date() was used, I figured it would be good to continue applying get_the_date filters so that they wouldn't break if sites were using them.

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Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

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[jwold]

@westonruter I'm still seeing the problem where it says 10 hours ago instead of 2 hours ago.

Steps I took:

1. Install Local with Prod link (I checked the downloaded file and it shows: Version: 2.0.2-alpha-20200917T192128Z-ee08122)
2. Set time stamp in Settings to Los Angeles
3. Run onboarding wizard: reader mode with legacy template
4. Create new post, set time to 2 hours previous, and publish
5. View in AMP mode: https://d.pr/i/CmNXzz

[jwold]

Followup: I had changed the time to 10 hours in the future instead of 2 in the past (PM was set by default in Gutenberg).

Once I fixed that the post shows as expected, saying 2 hours ago, but it raises the question of why it would say 10 hours ago if it's schedule for 10 hours in the future.

[jwold]

QA passed.

Re-tried following different steps, namely that I published immediately. The time stamp is now correct based on that action.