The easiest way to keep the 3 column layout...
Link your page to an actual post. That is, create a post that will be
your page. You can back-date it so it doesn't show up as new or you
can make it private (and still link to it) so it doesn't show up in
your list of posts. This way, it still looks great and you don't need
to edit any code. I've done something similar on my posterous by
linking pages to tags (
http://conardpeacock.com).
As for the post time and date go here:
http://posterous.com/theming/reference#block_Posts
In that section are the elements containing time and date. Find which
one you have in your theme and simply delete it if you don't want it.
I'm not sure you can simply make the date/time invisible to viewers
but visible to you. The timestamp will still be there, just not seen
so you should be able to restore it at any time.