How to get current date ,current month, year dynamically ib SAP BW reports

Drar All,

my requirement is to get current date ,current month, year dynamically in SAP BW reports?

for this we use Characteristic variable with customer exit-processing type on cal day, cal month,cal year, for cal day, cal month, cal year we have sap defined variables, why should we create again these variables using customer exit?

Thanks for helping,

Regards,

Narasimha