Showing old timestamp and then modifying it is bad idea (bad UX).
Best would be add message to message log with icon or text "Sending" (and without timestamp), then on error replace it with "Error - Try again" which would be clickable link,... and on success remove "Sending" and add result timestamp. I think I saw something similar in QIP and I always wanted it - also for accepting/rejecting files they have it as inline link.
But maybe it might be complicated for implementation? I have no experiences with RichEdit... (or time to learn or do it)
Anyway, it works with this delay with Scriver/TabSRMM too, so...