partial dates

data pr5;
  set pr4;

  *--- create dummy date for PREFL calculation;
    if substr(PRSTDTC,5,1) ne '-' then do;
       PAR_STD=cat(trim(PRSTDTC) || '-01-01');
    end;

    if substr(PRSTDTC,8,1) ne '-' and PAR_STD= "" then do;
       PAR_STD=cat(trim(PRSTDTC) || '-01');
    end;
    dumdate= input(PAR_STD, yymmdd10.);
    format dumdate date9.;

*--- create PREFL variable;
    if AENDT > . and AENDT <= TRTSDT then PREFL = "Y";
    else if dumdate ne . and dumdate < TRTSDT then PREFL = "Y";

run;