proc fcmp outlib=sasuser.funcs.trial;

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   function study_day(intervention_date, event_date);

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   if event_date < intervention_date then
      return(event_date - intervention_date);
   else
      return(event_date - intervention_date + 1);
   endsub;

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options cmplib=sasuser.funcs.trial;

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data _null_;
   start = '15Feb2006'd;
   today = '27Mar2006'd;
   sd = study_day(start, today);

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   put sd=;

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run;

http://support.sas.com/documentation/cdl/en/proc/59565/HTML/default/a003181727.htm

Proc FCMP Creating a Function and Calling the Function from a DATA Step

SAS