If the program involved always is run to refer to the previous month from the date of execution then you could stick this code inside the macro
data _null_;
sdate= intnx('month',today(),-1,'B');
edate= intnx('month',today(),-1,'E');
call symputx('begin_dt',sdate);
call symputx('end_dt',edate);
run;
Note that the values will be the numeric version of the SAS date value which should match for your uses UNLESS you are printing those directly in titles or such.
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