Dates are not stored in GMT for event instances

Here is perhaps a better example of the issue.

The date is being stored in the database as an incomplete ISO Timestamp (e.g. "2025-01-14T22:30:00"). Storing in this way instead of as a Unix Timestamp -- which assumes GMT -- doesn't give a developer enough context to trust the value.

Consider:

$event_instance->date->value =>  "2025-01-14T22:30:00"

\Drupal::service('date.formatter')->format(strtotime($event_instance->date->value), 'custom', 'M j, Y g:i A') => "Jan 14, 2025 10:30 PM"

To work around that, we either have to add the TZ offset to the ISO string before conversion, or calculate it afterwards.

Here's what I ended up with (which should be daylight savings compatible):

      $event_instance = $item['content']['#eventinstance'];
      // Extract the event instance date and URL.
      // The date is stored in the 'date' property of the event instance, but
      // that is offset by the timezone, so we need to format it correctly.
      $timezone = \Drupal::config('system.date')->get('timezone.default');
      $date = date_create($event_instance->date->value, timezone_open($timezone));
      $offset = date_offset_get($date);
      $datetime = strtotime($event_instance->date->value) + $offset;

This is very painful DX and should be corrected at the module level. Storing a full ISO date string or a Unix timestamp are both valid options.
Reference: https://www.php.net/manual/en/datetime.format.php

We're using the regular DateRangeItem field type, you can get a DrupalDateTime object for the start time and end time which can be used to format the date range for the desired time zone. For example:

$event_instance = $item['content']['#eventinstance'];
$start_date = $event_instance->date->start_date->format('M j, Y g:i A', ['timezone' => 'America/New_York']);
$end_date = $event_instance->date->end_date->format('M j, Y g:i A', ['timezone' => 'America/New_York']);

I just checked how this works in core and the data is stored in the same format when I add a daterange field to a node type. I checked with the ISO 8601 standard and indeed, the date format used by the datetime_range module is ambiguous. The dates should have a Z suffix to properly indicate this is a UTC time.

I think the most relevant core issue is ✨ [PP-1] Add ability to select a timezone for datetime field Postponed .