Indeed!

It works this way:

select

,....

,[version_prog]

,(CASE WHEN heure_passage BETWEEN '00:00:00' and '05:59:59' THEN (day(date_passage) -2)

ELSE (day([date_passage]) -1)

END) AS jourprod

FROM dbo.T_EOLT_BRUTE_FLOAT

Thanks Laurie, Asiaindian!