hi Matt,
you can do this via a Command object.
i.e. create a new report and instead of connecting directly to the proc, press the Add Command option.
once in your command create a Date parameter. then add your syntax referencing the date parameter and also a hard coded time.
e.g. using sap hana...
CALL STS.SELECTSTUFF({?StartDate},time'11:00:00.001')
where the startdate is a command based parameter and the time is just hard coded. the end user will enter a date based on a calendar, but not see the time prompt in the stored proc.
your syntax will vary though as the above is sap hana.
cheers,
jamie