Try with the logic below which should work in this case...
=
(
(
(ToNumber(SubStr(FormatDate(<Discharge Date> ,"HH:mm:ss") , 1 , 2 ) ) * 60 * 60 ) +
(ToNumber(SubStr(FormatDate(<Discharge Date> ,"HH:mm:ss") , 4 , 2 ) ) * 60) +
(ToNumber(SubStr(FormatDate(<Discharge Date> ,"HH:mm:ss") , 7 , 2 ))))
-
((ToNumber(SubStr(FormatDate(<Admission Date> ,"HH:mm:ss") , 1 , 2)) * 60 * 60 ) +
(ToNumber(SubStr(FormatDate(<Admission Date> ,"HH:mm:ss") , 4 , 2)) * 60) +
(ToNumber(SubStr(FormatDate(<Admission Date> ,"HH:mm:ss") , 7 , 2)))
)
) / 3600
+ (DaysBetween(<Admission Date> , <Discharge Date>) * 24 )