Gilbert -- a frustrating day.
I could not find a way to post a message with a date. And .. then I couldn't find a way to post a message and know the message id I posted .. The getdraft call is supposed to return a draft object with a message_id in it, but it always returns zero.
You probably saw Mark Fletcher's comment that the API can't do what I wanted it to do. Darn! I thought I could work around it by just posting all the old messages under the single user name "Imported message from old board" .. but ..even that isn't working.
Mark suggested converting to a format he could import. I'll investigate that route tomorrow...