PHP 8.4 deprecation: timezone_name_from_abbr() receives null for $isDST in TimezoneController::getTimezone()

Problem/Motivation

On PHP 8.4, the following deprecation notice is logged every time a user visits a page that triggers the timezone auto-detection AJAX request (e.g. /user/register with "Users may set their own time zone" enabled):

Deprecated function: timezone_name_from_abbr(): Passing null to parameter #3 ($isDST) of
type int is deprecated in
Drupal\system\Controller\TimezoneController->getTimezone()
(line 49 of web/core/modules/system/src/Controller/TimezoneController.php)

The deprecation does not crash the page, but it fills the database log with noise and will become a fatal error in PHP 9.

Steps to reproduce

Prerequisites

1. Install a clean Drupal 11.3.x site using the Standard installation profile, running on PHP 8.4.

Configure the site

2. Go to Administration → Configuration → Regional and language → Regional settings (/admin/config/regional/settings) and set "Users may set their own time zone" to "Users may set their own time zone". Save.
   
   
3. Go to Administration → Configuration → People → Account settings (/admin/config/people/accounts) and enable visitor registration under Who can register accounts. Save.
   
   

Trigger the bug

4. If you are not physically located in India, temporarily change your OS timezone so that Intl.DateTimeFormat().resolvedOptions().timeZone returns Asia/Kolkata (macOS may report this as the legacy alias Asia/Calcutta — both refer to the same timezone). On macOS: System Settings → General → Date & Time → disable "Set time zone automatically using your current location" → set Closest city to "Kolkata - India". The Time zone field will update to India Standard Time.
   
   This is the key condition: India Standard Time (UTC+5:30) has no Daylight Saving Time, which causes timezone.js to send an empty string for the DST parameter. Most Western timezones observe DST and send 0 or 1, which matches the route constraint and avoids the bug.
   
   
5. Open a browser (or a new private window) and visit /user/register.
6. Go to Administration → Reports → Recent log messages (/admin/reports/dblog), filter by type php. The deprecation notice will appear. It is triggered because timezone.js sends a request to /system/timezone/0/19800/ with an empty last segment for {is_daylight_saving_time}, which does not match the route constraint \-1|0|1, so Symfony falls back to the route default NULL. Click the entry to see the full message and stack trace.

Proposed resolution

The cleanest fix is a single-line change in core/modules/system/system.routing.yml: set the default value of is_daylight_saving_time to -1 instead of NULL.

-1 is the documented sentinel value for timezone_name_from_abbr() meaning "do not consider DST". The same pattern is already used for the offset parameter in the same route (offset: -1). Changing the default means the controller always receives a valid int whether the URL segment is present or absent.

 system.timezone:
   defaults:
     offset: -1
-    is_daylight_saving_time: NULL
+    is_daylight_saving_time: -1

This also makes core/misc/timezone.js semantically correct without any JS change: the empty string it sends for no-DST timezones falls through to the default, which will now be -1 rather than NULL.

Optional hardening — also fix core/misc/timezone.js to send -1 explicitly instead of '', so the URL segment is always present and valid:

- var isDaylightSavingTime = '';
+ var isDaylightSavingTime = -1;

Remaining tasks

• Confirm the fix on Drupal 10.x (likely affected on PHP 8.4 as well)
• Add a regression test

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

None required.

PS: AI was used to review and re-word my original description