I know this has been going on for sometime, since 1.8.x I believe and is present in the 1.9.x builds too, to save me having to tell everyone hosted here:

Under some conditions, such as editing a post under a locale that's not m/d/y the system will return invalid date.

I avoid this by changing my user to English (US), this puts the post by default as English (US). However you can easily change the language on the entry itself back to English (UK) when composing your post without being tripped up by the invalid date error.