SELECT T.startdate,employeename FROM Employee
LEFT JOIN TimeSheet T ON T.EmployeeID!=EmployeeID
WHERE active =1 and convert(VARCHAR, T.startdate, 111) = '2012/02/17'
order by employeename
OR
SELECT Convert(datetime, '2012/02/17') AS startdate,
employeename FROM Employee
WHERE active =1 and EmployeeID NOT IN (SELECT EmployeeID FROM TimeSheet WHERE convert(VARCHAR, startdate, 111) = '2012/02/17' ) order by employeename