sql server 2005 :how to put not null constraint on a column depending upon value in other column?

in a table two of columns are billable(bit),billabledate(datetime).i want billable date to be not null if billable is not null.

From stackoverflow

• I'd try adding a trigger to the table, on after insert and after update, to enforce that constraint. Check the billable value, and block insert/update in case it is not null and the billabledate is null.

• Add a check constraint:

  CHECK (billable is not null and billabledate is not null) OR (billable is null)

• You need a Check Constraint

  ALTER TABLE dbo.Table WITH NOCHECK
  ADD CONSTRAINT CK_Table_BusinessRule CHECK (Billable IS NOT NULL AND BillableDate IS NOT NULL)

  http://msdn.microsoft.com/en-us/library/ms179491(SQL.90).aspx