Steps to reproduce the issue

These are the instructions for 4.0:

• Go to System > Site Templates
• Select Cassiopeia Details and Files
• Press New File from the toolbar
• From the left side select the html/mod_menu folder
• Set File Name to exactly this: index-20210203-135009
• Set File Type to .php
• Press Create

This is a code snippet that showcases the issue:

$format = 'Y-m-d';
$value = '2021-02-03';
$validDate = JFactory::getDate()->createFromFormat($format, $value);
echo $validDate->format('d.m.Y');

Expected result

File created. success message and the newly created file being edited in the view.

The code snippet above should show 03.02.2021

Actual result

Error shows up DateTime::setTimezone(): Argument #1 ($timezone) must be of type DateTimeZone, null given in both cases.

System information (as much as possible)

PHP 8.0.1

Additional comments

The PHP changelog notes this:
Fixed bug #60302 (DateTime::createFromFormat should new static(), not new self()).

• On PHP < 8, createFromFormat returns a DateTime object.
• On PHP 8+, createFromFormat returns the extending class, in this case \Joomla\CMS\Date\Date

Perhaps changing this call https://github.com/joomla/joomla-cms/blob/4.0-dev/libraries/src/Date/Date.php#L339-L342

if ($local == false)
{
	parent::setTimezone($this->tz);
}

To something like:

if ($local == false && $this->tz !== null)
{
	parent::setTimezone($this->tz);
}

Might fix this without causing other issues.