If you have a date variable then it is already a number, then number of days since 01JAN1960. Just display it using a different format.
If you have '01SEP2015' in a character variable or input file then you can use the INFORMAT of DATE9 to convert it into a SAS date value.
datestr='01SEP2015';
dateval=input(datestr,date9.);
format dateval date9. ;