Another difficulty for the Oxfordians is that the 17th Earl of Oxford died in 1604.Many of the greatest plays were produced after this date.Macbeth, for example, dates from 1606-1607 and The Tempest from 1611. A great deal of careful work has confirmed these dates, and most Oxfordians reluctantly concede that de Vere's death preceded the appearance of these plays.But the Oxford camp persists in their position.They argue that de Vere wrote them before he died and that they were brought out as needed for performance.In addition, the texts of many Shakespeare plays contain references to events after 1604.The Oxfordians say someone must have added contemporary references to make the plays look timely.