When you set the When Expired property to be a date in the past, the user interface (UI) will show Expired text, and the event will come back with a 20 for the status property. This is because the status has expired due to the display date being in the past.
Important Note:
Expired events are marked as such by an asynchronous process that runs periodically. Therefore, there may be delays in the status change, but such events will not appear in the lists of published events.