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You can use the date control and set this to time, this will give you the hh:mm setting and the control checks the input. Put a check box next to the control with the header Not Known. If the user ticks the check box in the time control or enters a time that forces the check to be ticked in the time control have an event set the Not Known tick box to unticked. In your document you can check the time control, if not ticked set the value to now known.
Erwin Oosterhoorn BSc (Hons), Analyst/Programmer
"They can tak' oour lives, but they cannae tak' oour troousers!"