SAS provides date interval functions for handling date operations without having to refer to date values internal representation. For shifting dates, the function is INTNX :
data frequencydate;
format weekStart yymmdd10.;
start_date = '01Nov2018'd;
do i = 0 to 21;
weekStart = intnx("week", start_date, i, "sameday");
output;
end;
drop start_date;
run;
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