Remove mysql2date() usage from core

[Rarst]

mysql2date() was originally meant from processing times as stored by WP in database.

Unfortunately its design is very limited because both time zone of input and output is ambiguous. It is interchangeably used for local and UTC times.

As implementation detail that meant that it would produce incorrect output for formats including time zones for local time inputs. Time zone would happen be UTC from WP core setting PHP time zone to UTC on load.

In 5.3 release this behavior was flipped to produce correct local time output for local time input, as considerably more common. Accordingly now UTC input produces incorrect output for formats with time zone.

While the function is common and familiar, it is hardly irreplaceable and its design is so limited it seems to be unsalvageable.

I propose we move towards eliminating its use in core and eventually formally deprecating it.

[coquardcyr]

Inside the core, we have the following usages from the function:

• ​Line 150 of src/wp-content/themes/twentytwenty/classes/class-twentytwenty-walker-page.php
• ​346 of src/wp-admin/includes/class-wp-ms-users-list-table.php
• Lines ​248, ​627 and ​630 of src/wp-admin/edit-form-advanced.php
• Lines ​103 and ​104 from src/wp-includes/feed-atom-comments.php
• ​Line 79 from src/wp-includes/feed-rdf.php
• ​Line 96 from src/wp-includes/feed-rss2.php
• ​Line 96 src/wp-includes/feed-rss2-comments.php
• ​Line 220 of src/wp-admin/edit-form-blocks.php
• Lines ​326 to ​331 of src/wp-admin/includes/template.php
• Lines ​469, ​470 and ​477 of src/wp-includes/class-wp.php
• ​Line 217 of src/wp-includes/class-walker-page.php
• Lines ​479 and ​503 of src/wp-admin/includes/class-wp-ms-sites-list-table.php
• ​Line 583 of src/wp-admin/includes/export.php
• Lines ​538 and ​551 of src/wp-includes/ms-functions.php
• Lines ​2257 and ​2850 src/wp-admin/includes/ajax-actions.php
• Lines ​2673 and ​2689 of src/wp-includes/class-wp-customize-manager.php
• Lines ​590 and ​1086 of src/wp-includes/comment-template.php
• ​Line 1813 of src/wp-includes/class-wp-editor.php
• Lines ​900 and ​901 of src/wp-includes/comment.php
• Lines ​880 and ​892 of src/wp-includes/class-wp-xmlrpc-server.php
• Lines ​4717 and ​4718 of src/wp-includes/class-wp-query.php
• Lines ​2049 and ​2328 of src/wp-includes/link-template.php
• Lines ​2119 and ​2500 of src/wp-includes/general-template.php
• ​Line 1695 of src/wp-admin/includes/media.php

-​Line 1274 of src/wp-admin/includes/upgrade.php
-​Line 7418 of src/wp-includes/functions.php

• ​Line 4157 of src/wp-includes/media.php

[pbearne]

What should we replace this with?

[sukhendu2002]

I think we can use wp_date() for most cases. It handles timezones properly.

For displaying dates to users, we'd do something like:

$datetime = date_create($date_string, wp_timezone());
wp_date($format, $datetime->getTimestamp());

For UTC time outputs (like in feeds), gmdate() works well:

$timestamp = strtotime($date_string);
gmdate($format, $timestamp);

For comparing dates, it's cleaner to just create DateTime objects and compare them directly rather than converting to timestamps.