Is the 'last message received' text above the message box no good?
If it's not possible to add the timestamp to handwritten messages, then I think a perfect script would be for it to add the current time to the tablet PC user's name, which would be updated every minute. Then when they send a handwritten message, the time the message was sent will be in their name.
Closest I can find is
Date+Time but it adds it to the users personal message. Maybe someone could customise it so it can be used in the display name?
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