In pure SQL you need a decode:
SELECT
Decode(ZEUS.SPEC_ORD.SPEC_ORD_STAT,
'EEA',ZEUS.SPEC_ORD.SPEC_ORD_STAT_CRBD - ZEUS.SPEC_ORD_STAT_HIST.STAT_CHANG_DATE,
NULL)
FROM
ZEUS.SPEC_ORD,
ZEUS.SPEC_ORD_STAT_HIST
WHERE
( ZEUS.SPEC_ORD.SPEC_ORD_NUMB = ZEUS.SPEC_ORD_STAT_HIST.SPEC_ORD_NUMB )