format_date() function converts date to user timezone regardless $convert=false

[metalefty]

Prerequisites

• I have searched for duplicate or closed issues
• I can recreate the issue with all plugins disabled

Describe the issue

Here's the background
#7352 (comment)

Expected behaviour:
format_date() does not convert the timezone of the given date.

format_date('Tue, 8 Oct 2024 07:12:23 +0000', 'u', false); # => 2024-10-08T16:12:23+09:00

Actual behaviour:
format_date() always converts the timezone of the given date to user timezone.

format_date('Tue, 8 Oct 2024 07:12:23 +0000', 'u', false); # => 2024-10-08T07:12:23+00:00

What browser(s) are you seeing the problem on?

Chrome, Edge, Firefox, Safari, Other

What version of PHP are you using?

8.1.29

What version of Roundcube are you using?

1.6.9

JavaScript errors

No response

PHP errors

No response

[pabzm]

I don't quite understand you examples: what you show as "expected" behaviour does change the timezone, doesn't it? Did you accidentally swap the two code lines?
(Also the $format 'u' should be 'c' I think.)

Regardless I can confirm that the $convert argument doesn't do what it claims.

It is called from core code only in program/actions/contacts/index.php:1353, and additionally in the plugin 'managesieve' (several times in plugins/managesieve/lib/Roundcube/rcube_sieve_vacation.php). So I guess the danger to break important things is rather low.
Unfortunately we don't have any tests for that method yet.

@metalefty Do you by chance feel competent to propose a fix? Also starting one and asking for help if you get stuck would be helping a lot already. I guess not converting the input into timestamps and maybe starting to use IntlDateFormatter would be good first steps.