TYPO3 Core

I can confirm this with TYPO3 10.4 too (had the same problem now),

There's a comment above this line

https://github.com/TYPO3/TYPO3.CMS/blob/430d6b5780c4a24a164cc07d3cc1d7569f4f92e8/typo3/sysext/core/Classes/DataHandling/DataHandler.php#L1684

// We store UTC timestamps in the database, which is what getTimestamp() returns.

but then the DateTime is created without specifying the UTC timezone ;-)

The patch fixes this issue:

--- Classes/DataHandling/DataHandler.php.bak    2020-11-16 18:09:28.000000000 +0100
+++ Classes/DataHandling/DataHandler.php    2020-11-16 18:09:55.000000000 +0100
@@ -15,6 +15,7 @@

 namespace TYPO3\CMS\Core\DataHandling;

+use DateTimeZone;
 use Doctrine\DBAL\DBALException;
 use Doctrine\DBAL\Driver\Statement;
 use Doctrine\DBAL\Platforms\PostgreSqlPlatform;
@@ -1869,7 +1870,7 @@
                 // Convert the date/time into a timestamp for the sake of the checks
                 $emptyValue = $dateTimeFormats[$tcaFieldConf['dbType']]['empty'];
                 // We store UTC timestamps in the database, which is what getTimestamp() returns.
-                $dateTime = new \DateTime($value);
+                $dateTime = new \DateTime($value, new DateTimeZone('UTC'));
                 $value = $value === $emptyValue ? null : $dateTime->getTimestamp();
             }
         }

Wolfgang Klinger wrote in #note-2:

// We store UTC timestamps in the database, which is what getTimestamp() returns.

but then the DateTime is created without specifying the UTC timezone ;-)

This refers to the fact that a \DateTime uses the configured timezone and a unix timestamp is in UTC by definition.
So a new \DateTime('1.1.1970 01:00') in timezone MEZ will yield 0 for getTimestamp().
That is what the comment actually tries to convey.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/66670

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/66670

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/66670

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/66670

Patch set 6 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/66670

Patch set 1 for branch 10.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/67200

Patch set 1 for branch 9.5 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/67201