Crystal Reports Parameter Question

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     I haven't had the need to use many parameters in my reports and I am having a problem getting one to work. I have a formula in my report that looks at hours in a pay period. I need a parameter that lets them choose wether they want to look at the current or previous one. Here is the formula that I have in the select statement that looks at the transactions in the previous pay period:

{VP_TOTALS.APPLYDATE} in {VP_TOTALS.PREVPAYPERIODSTART}  to {VP_TOTALS.PREVPAYPERIODEND

and this is the one that looks at the current one:

{VP_TOTALS.APPLYDATE}  in {VP_TOTALS.CURRPAYPERIODSTART}  to {VP_TOTALS.CURRPAYPERIODEND}

for some reason I cant get my brain in gear to set up a parameter that works that allows them to choose. 

Thanks
KevV 