how to use this subquary : i want to see if time is below 0 with this quary

    select *
    from voorwerp
    WHERE (Select (((Cast(((DATEADD(dd, looptijd, datum)) - getdate()) as float) * 24.0)*60.0)*60.0) AS tijd from voorwerp) <= 0

i get this error :

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.
