Is such a cheque a valid negotiable instrument?

Yes.

In the USA it does but the bank or whoever gives you the check is not obligated to hold it until the date anymore. Banks cash post dated checks before the date all the time anymore.

Some tellers will catch that it is post dated and not, but once it is cashed they have a oh well too bad attitude. At least in my state they do that. Post dated checks really aren't legal in the eyes of the bank at least in the sense of if there is a dispute, you the one that post dated usually won't win.