Oracle 18c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
Select
      to_char(time'0:0:0'+numtodsinterval((7.5 + 0.75 - 8 ),'hour'),'hh24:mi')
from dual


TO_CHAR(TIME'0:0:0'+NUMTODSINTERVAL((7.5+0.75-8),'HOUR'),'HH24:MI')


00:15


Select to_char(
         time'0:0:0' + numtodsinterval(8 - (7.5 + 0.75),'hour'),
         'hh24:mi'
       )
from   dual


TO_CHAR(TIME'0:0:0'+NUMTODSINTERVAL(8-(7.5+0.75),'HOUR'),'HH24:MI')


23:45


SELECT numtodsinterval(8 - (7.5 + 0.75),'hour') AS interval,
       REGEXP_REPLACE(
         numtodsinterval(8 - (7.5 + 0.75),'hour'),
         '([+-]?)(\d+) (\d+):(\d+):(\d+\.?\d*)',
         '\1\3:\4'
       ) AS hhmm
FROM   DUAL;


INTERVAL
HHMM


-000000000 00:15:00.000000000
-00:15