Fixes #39984.

The date/time being displayed in the Date block is currently using gmdate():

This is incorrect. For comparison, refer to core's get_post_time() function as used by get_the_date() as used in classic themes. It uses wp_date() as follows:

$time = wp_date( $format, $datetime->getTimestamp(), $gmt ? new DateTimeZone( 'UTC' ) : null );

When the $timezone parameter is not passed to wp_date() then it uses the current timezone as specified in the blog's settings.

Testing

I added a Date block in a row in the Single template in Twenty Twenty-Five:

I also have my timezone set to America/Los Angeles:

The current time for me right now is 22:35 (10:35pm) on August 31st. In the block editor, I see this reflected correctly when showing the template in the block editor:

However, if I view the post on the frontend, I see September 1st, since that is the current date in UTC/GMT:

When I switch to the code in this branch so that wp_date() is used instead, then I see the expected date of August 31st: