I am sure that you found a work around by now, but this is what I do:
have the date parameter by a string and allow the user to enter the string.
if you want the user to select from a list, I would use a command object and union the date selection (converted to a string) with ' All'. The space allows it to sort to the top of the list.
in the selection formula, I would replace the ' All' with a value that would be small or large enough to encompass all the data
HTH
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