@korenevskiy Will this solve issue #41265 ? If so, please link to the issue at the top of the description of this PR (Pull Request for Issue #xxxxx).

This will not work, there were other PR somewhere around.

strftime format https://www.php.net/manual/en/function.strftime.php and DateTime format https://www.php.net/manual/en/datetime.format.php is incompatible.

Doesn't work - sorry

It was the PR that introduced this very dodgy helper function (for better or worse)

@wilsonge That helper method is not used by Calendar Form Field, so it is unrelated. It was created to address the deprecated warnings HTMLHelper calendar in method with with the most common DateTime format parameters (until we a better solution).

in site https://www.php.net/manual/en/function.strftime.php
Below is a method to support the old code.
https://github.com/alphp/strftime/blob/master/src/php-8.1-strftime.php
This function is very large.

$format = HTMLHelper::strftimeFormatToDateFormat($this->format);
$this->value =  (new DateTime($this->value))->format($format);

In the corrected format, we use DateTime object. The function also converts the format to a new one for DateTime.

Let's create a static method strftime for the JDate class.

From https://github.com/alphp/strftime/blob/master/src/php-8.1-strftime.php
And let's call it obsolete - deprecated before Joomla 5
It's funny, but it's the right decision.

@korenevskiy The right solution is adding filterformat to your field

<field name="date_start" type="calendar" default="NOW" filterformat="Y-m-d"
label="MOD_MULTIMODULE_FIELD_FILTERDATESTART_LABEL"     showon="date_is:on"  
showtime="false"  singleheader="true" />   

Then it will work well. No warnings anymore.

Then it will work well. No warnings anymore.

But where is backward compatibility? The CALENDAR field should work without the filters attribute.

But where is backward compatibility? The CALENDAR field should work without the filters attribute.

It was PHP, not Joomla! introduced that backward incompatible change and Joomla! already provided you the solution with the undocumented filterformat attribute, you will just have to adapt your code to work with the changes introduced by PHP.

There is no reliable way to to convert all the format parameters supported by strftime to format parameters uses by DateTime class, so you cannot do the magic conversion (there are some format parameters in strftime has no equivalent format in DateTime class).

We need to ensure backward compatibility with older codes. At the same time, support PHP 8.2. At the same time, the format field should fully support backward compatibility, with full support strftime()
@joomdonation According to the PHP.net website. The function laid out with full support for the old function.
There's a link at the bottom of the page.

Let's create a static method strftime for the JDate class.

Everyone vote

Colleagues, I added a static strftime() method to the Date class

This method works

$this->value = JDate::strftime($this->format, strtotime($this->value));

No matter how stupid this way of solving the problem would not be. This is the only way that meets all the requirements. There are no other ways.

@korenevskiy do not waste your time, re-implementing of strftime is not a correct way to go.

@Fedik

Of course that is wrong. For example, Joomla 5 should support PHP 8. Which means We can't release a release with an error popping up. There is another option to abandon the old format support. But it's also a backward compatibility violation. I wouldn't waste time if I didn't weigh the options. My option is very bad, BUT it is the only one that will allow you to maintain backward compatibility without errors. And already with the release of Joomla 6, this method can be removed.

There is another option, to pretend that nothing is happening. Meanwhile, 100% of Joomla 4 users on PHP 8.1 will display an error.

There is another option. We simply write the contents of this function into the CalendarField class. This will allow, as you say, not to create new functions. But even such a swollen Calendar FIeld is even worse than my version.

Each participant will believe that there will be a magic way of 3 lines that will solve all the problems, and will think that someone else will guess for sure. I read the contents of this function. And I will say that it will not be possible to replace it with 3 lines. There is absolutely no way to replace it.

Joomla 5 should support PHP 8. Which means We can't release a release with an error popping up

It works. It not an error, it a warning for developers.

A proper fix:

Introduce a new property datetime-format=""
The calendar class checks for this property, and when it exists then use it for value display and for value filter,
Othervise it continue to use strftime.

However, this is require changes in calendar.js, new lang constant, and review all our forms.
If you feels brave enough, you are welcome to do it.

Otherwise, any hacking around strftime is a waste of time.

Introduce a new property datetime-format="" The calendar class checks for this property, and when it exists then use it for value display and for value filter, Othervise it continue to use strftime.

@Fedik I think we have filterformat attribute for that purpose already https://github.com/joomla/joomla-cms/blob/4.3-dev/libraries/src/Form/Field/CalendarField.php#L238 . Shouldn't this PR be closed ?

I think we have filterformat attribute for that purpose

That for filter, I have suggest to use new attribute datetime-format="" for display, this should use DateTime style format instead of strftime.

But in general the Calendar need a bigger fix, see #41444
A deprecated strftime is a last thing I would worry ?

This pull request has been automatically rebased to 4.4-dev.

We discussed this PR in the maintainers meeting and would suggest the following.
include the function with composer based on https://github.com/alphp/strftime
and detect if the function exists in the bootstrap.php, if it's missing load the polyfill and include the function.

As Harald said,
please use the composer dependency and make a PR against 4.4

As Harald said, please use the composer dependency and make a PR against 4.4

Explain what it means to make a dependency on the compositor in our case.