[KBK] observes:

At one point, I had a customer who wanted an entry field that would validate a time of day (HH:MM:SS); actually, since the application was for video processing, the time also included a frame count within the second, but that's not really relevant to the discussion.  The customer found "validate-on-focus-out" to be unacceptable: he wanted the field NEVER to have invalid content.  What the customer (and, surprisingly, the users) actually liked was to put a sample time in the field, and 
set up the keyboard bindings to function in overtype mode.  The key bindings got to be a bit complicated, since they were dependent on the location of the entry cursor.

Here's the code, in case anyone else finds the idea useful:
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(ugh, I've found that it's too big for my browser to accept in the
text area.  I promise to find another place to put it.)

''[RS] is happy again that he likes tiny code snippets which fit between thumb and index finger..;-)''