I guess I am just confused about the purpose of the parameter.
If it is a date, why not just the 'today' value. If the only thing that the user is editing is the time, is your report time sensitive for the date, or do you just want all the values of a particular date?
you could make the parameter nullable and if null use the current datetime value or date, whichever is desired.
again, I'm just confused as it seems that you are doing a lot of work for what seems like a simple parameter...though it seems that I miss the obvious sometimes.