Another option is to have the date range parameters in the command in the subreport, but to also create them and the {?Relative Date} parameter in the main report. You can then use two Crystal formulas to calculate the start date and the end date based on the {?Relative Date} parameter. Use these two formulas to link to the command parameters in the subreport. That way you don't have to do a more complicated "where" clause in the command, you just have to account for the date range.
-Dell