crystal report formulas issue[urgent] CRYSTAL XI

I have two formulas set up from the formula workshop.

First Day :Minimum(MonthToDate);

and Last Day: dateserial(year(CurrentDate),month(CurrentDate)+1,1)-1

I inserted both Formulas into the report and yet the result is a blank report. Do I need to set these formulas equal to the date parameters I have existing in the report. In this case @start date and @end date..so that if the user does not select the dates or type in a date that the formulas will be used ?

How would I do that ?