If START_DATE and END_DATE are character values, those need to be converted to numeric SAS date values before calling DATDIF. One way to convert a character value to a numeric value is to use the INPUT function.
Here is a small program that uses the INPUT function and the YYMMDD. informat to convert character date values into numeric date values.
[pre]
data in;
start_date = '20100201'; end_date = '20100204'; output;
run;
proc sql;
create table out
as select
datdif(input(start_date, YYMMDD8.),
input(end_date, YYMMDD8.),
'ACT/ACT') as datdif,
start_date,
end_date
from in;
quit;
[/pre]