Calculate the date that is N workdays from a given date

CREATE FUNCTION dbo.ufn_WorkdaysFrom(@workdays int, @start date)
RETURNS DATE
AS
BEGIN
    RETURN (SELECT TOP 1 date
            FROM Calendar P
            WHERE date > @start AND workday = 1 AND (
                SELECT COUNT(NULLIF(C.workday,0))
                FROM Calendar C
                WHERE C.date > @start AND C.date <= P.date
                ) = @workdays)
END