Your code does not match your presented data (different variable names).
Your code will work with matching data, see here:
data want;
input _date_start :$10.;
date_start = input(_date_start,ddmmyy10.);
format date_start date9.;
cards;
21-02-2016
17-12-2015
23-02-2016
08-05-2016
;
run;
proc print data=want noobs;
run;
Result:
_date_ date_
start start
21-02-2016 21FEB2016
17-12-2015 17DEC2015
23-02-2016 23FEB2016
08-05-2016 08MAY2016
If this does not solve your problem, post your log ({i} button), and example data (data step with datalines, see my example and my footnotes).
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