Calculations using both date and date/time fields

I have two fields, lets call them Field-1 which is a Date type and Field-2 which is a Date/Time type.  I need to calculate the difference between these two fields in hours or fractions of days.  However, the Date field doesn't have time.  It is fine if the solution involves Field-1 defaulting to midnight.  I tried simply subtracting Field-1 from Field-2 but that doesn't work.  Tried using the CAST function to CAST Field-1 as a TIMESTAMP.  That doesn't work.  Tried using the TIMESTAMPDIFF function which doesn't work because they aren't both Date/Time fields.  Does anyone have a set of conversions and/or