If Formula require to calculate No of Days

Hi,

I have four dates in my work sheet, among them 2 are fixed and two variables. The data is as follows:

Date of Purchase (DP) = variable
Date of Sale (DS) = variable
Period begin date (PBD)= 30-JUN-09
Period end date (PED)= 30-APR-10
Result (Number of Days)

Conditions are as follows:

if (DP and DS) is not null and (DP and DS) < PBD then Result= 0
if DP < PBD and DS is null then result = PED-PBD
if (DP and DS) > PBD then result = DS-DP
if DP > PBD and DS is null then Result = PED-DP

please help,

Really appreciated

Regards,