This PR was merged into the 6.1 branch.

Discussion
----------

[HttpKernel] Fix timezone influenced tests

| Q             | A
| ------------- | ---
| Branch?       | 6.1
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | #45589 (comment)
| License       | MIT
| Doc PR        | -

`DateTimeValueResolverTest` fails depending on the default timezone and the current time.

The culprit is the combinaison of `strtotime` + `new DateTime`.
https://github.com/symfony/symfony/blob/24304ad77874f03f9971fd9e29c0ef0776b0f5b9/src/Symfony/Component/HttpKernel/Controller/ArgumentResolver/DateTimeValueResolver.php#L67-L69

```
There were 3 failures:

1) Symfony\Component\HttpKernel\Tests\Controller\ArgumentResolver\DateTimeValueResolverTest::testFullDate
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'2012-07-21'
+'2012-07-20'

/home/runner/work/symfony/symfony/src/Symfony/Component/HttpKernel/Tests/Controller/ArgumentResolver/DateTimeValueResolverTest.php:53

2) Symfony\Component\HttpKernel\Tests\Controller\ArgumentResolver\DateTimeValueResolverTest::testCustomClass
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'2016-09-08'
+'2016-09-07'

/home/runner/work/symfony/symfony/src/Symfony/Component/HttpKernel/Tests/Controller/ArgumentResolver/DateTimeValueResolverTest.php:100

3) Symfony\Component\HttpKernel\Tests\Controller\ArgumentResolver\DateTimeValueResolverTest::testDateTimeImmutable
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'2016-09-08'
+'2016-09-07'

/home/runner/work/symfony/symfony/src/Symfony/Component/HttpKernel/Tests/Controller/ArgumentResolver/DateTimeValueResolverTest.php:116
```

Commits
-------

9a0e99c Fix timezone influenced tests

… (codedmonkey)

This PR was merged into the 6.1 branch.

Discussion
----------

[Serializer] Add information about DateTimeValueResolver

Adds documentation for symfony/symfony#45589.

I think it would be a good idea to explain how to use the `MapDateTime` attribute, but I opted to keep it brief.

Commits
-------

9c910d7 Add information about DateTimeValueResolver