This is causing strange issues, among others, it is confusing Simpletest, while the test can be found and executed, the results screen is messed up and it's not possible to re-execute the test.
Looks like the naming pattern is Datetime for datetime.module and DateTime for the Core component stuff. So the file should be reanamed to DatetimeFieldTest.php.
This should be trivial when using a case sensitive file system so tagging as Novice. You might not want to try this if you're using something archaic like Windows, though ;)