Hi esmi - thanks so much for the response!
I did try the suggestion to add the define statement to my wp-config.php file,
unfortunately it didn't work. Still no response when I click on "edit" to change the post timestamp.
I carefully checked all my plugins before upgrading to ensure compatibility, and those that weren't, I ditched. What I have left are all critical plugins so I can't try disabling them until late at night as the blog gets a good amount of traffic daily.
But what seems odd is that this feature worked perfectly before upgrading, and now doesn't - I'm sure my server configuration hasn't changed, so I don't really think it's related to the cron process, and the Theme has been carefully checked also, although I don't see what the Theme would have to do with a core function of the back end, that is scheduling posts, unless I'm missing something about the interaction of Themes with this feature of scheduling posts.
Anything else I could try?
P.S. I also enjoyed the long explanation by otto42 of the cron process - I guess that makes me a masochist too, but we are lucky to have him moderating these forums!