Add a dropdown text value that can be turned on or off, like "Anytime" or "None Selected".  Or even better allow the programmer to change this value.  One example where it is needed:  In an Air Search on a travel site, there is a time value that can be sent to the reservation systems for finding flights.  It is not mandatory, but it does change the results provided if time is included.  In the current implementation, there is no way for a user to not choose a value.