Finding last Sunday and going 4 weeks backward every week in SAS - date

Question

I have a SAS job that runs every Thursday, but sometime it need to run on Wednesday, and maybe Tuesday evening. The job collects some data in 4 week intervals up until the closest Sunday. For example, today we have 19Mar2015, and I need data until 15Mar2015.
data get_some_data;
set all_the_data;
where date >= '16Feb2015' and date <= '15Mar2015';
run;
Next week I have to manually change the date parameters too
data get_some_data;
set all_the_data;
where date >= '23Feb2015' and date <= '22Mar2015';
run;
Anyway I can automate this?

Answer 1

I'll expand on the suggestion from #user667489 as it could take you a while to work it out. The key is to use the week time interval, which by default starts on a Sunday (you can change this with a shift index, read this for further details)
So your query just needs to be :
where intnx('week',today(),-4)<date<=intnx('week',today(),0);

Answer 2

Use the INTNX function to regress the date back to last Sunday:
data get_some_data;
set all_the_data;
lastsun=intnx('week',today(),0);
/*where date >= '23Feb2015' and date <= '22Mar2015';*/
where date between lastsun-27 and lastsun;
run;

Answer 3

You can try getting the last sunday date using weekday function and then using INTNX get the 4 week back date from that sunday date. Check the below ref code :
data mydata;
input input_date YYMMDD10.;
/* Loop to get the last sunday date, do the processing
and get out of loop */
do i =0 to 7 until(last_sunday_date>0);
/* Weekday Sunday=1 */
if weekday(sum(input_date,-i))=1 then do;
last_sunday_date=sum(input_date,-i);
/* INTNX to get the 4 week back date */
my_4_week_start=intnx('week',last_sunday_date,-4);
end;
end;
format input_date last_sunday_date my_4_week_start yymmdd10.;
datalines4;
2015-03-01
2015-03-07
2015-03-14
2015-03-21
2015-03-28
2015-04-05
2015-04-13
2015-04-20
;;;;
run;
proc print data=mydata;run;
let me know if this helps!