I wouldn't provide a date. The regulation allows you to not investigate it for missing information. Keep it at that ...."We cannot investigate your claim because you did not provide the following information..." If they want to provide the info, add it to the original file with documentation of when you received it, or have them provide a new dispute notice (complete a new form if you are using one).
Giving them a "get it to us by this date" clause, ties you up with having to go back later and tie up any loose ends on the outstanding dispute. I would just send them a letter telling them that I need more info and be done with it.
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I can't herd the cats anymore, so I just set up the electric fences and let them fry when they stray out of bounds.