I really like the link you've added to the php.net page. To make this shorter (because it's always a challenge to mention these edge-case details without cluttering things up), how about we:

a) link to http://php.net/manual/en/function.date.php#refsect1-function.date-changelog (so that it goes right down to that note)
b) Omit the note starting with "The valid range of a ....".

And with that, I think I love it!