You need to create a macro variable with the date especially if the date will change depending on when you run the code. Then reference that variable in the export statement. The code below grabs today's date.
%let numdate = %sysfunc(today(), yymmddn8.);
PROC EXPORT DATA= WORK.ARG_REPORT
OUTFILE= "/dwdata/sas_data/REPORT_&numdate..csv"
DBMS=CSV REPLACE;
RUN;