Oracle calculation performance

How does Oracle handle repeated calculations?  Is it evaluated once or is it evaluated each time it is encountered?

For instance in a case statement creating buckets for counting

CASE
    WHEN A Date - Discharge Date < 31 THEN '000-030'
    WHEN A Date - Discharge Date < 61 THEN '031-060'
    WHEN A Date - Discharge Date < 91 THEN '061-090'
    ELSE    '091+'
END

Can this be rewritten in the other CASE form as

CASE A Date - Discharge Date
    WHEN ???? THEN  '000-030'
    ...
   ELSE '091+'
END

I realize the SQL is not in Oracle SQL format.  My question is on the substance as the tool I use will put it in Oracle format when the query is built.
I know some databases will calculate the A Date - Discharge Date once and others would in this case calculate it 3 times.  This is just an example. My real statement I am working with has the calculation repeated 14 times thus my concern for speed

mlmcc