select '21.00' - '21.30' from dual

Those aren't times! They're numbers!

If you want to do time arithmetic, either

Make them dates

When you subtract dates, the result is the number of days difference. So assuming you want the result as the number of hours difference including fractions, multiply up by 24, rounding as needed:

select round((to_date('21:00', 'hh24:mi') - to_date('21:30', 'hh24:mi')) * 24, 2)
from   dual;

ROUND((TO_DATE('21:00','HH24:MI')-TO_DATE('21:30','HH24:MI'))*24,2)  
-0.5

Make them day to second intervals

select to_dsinterval ('0 21:00:00') - to_dsinterval ('0 21:30:00') 
from   dual;

TO_DSINTERVAL('021:00:00')-TO_DSINTERVAL('021:30:00')  
-00 00:30:00.000000  

This time to get the fractional hours, extract the minutes and divide by 60:

select extract( minute from to_dsinterval ('0 21:00:00') - to_dsinterval ('0 21:30:00') ) / 60
from   dual;

EXTRACT(MINUTEFROMTO_DSINTERVAL('021:00:00')-TO_DSINTERVAL('021:30:00'))/60  
-0.5 

If your times can be more than an hour apart you'll need to extract the hours out and add these on too.