One of the most vexing problems in historiography is dating an event when the usual sources offer conflicting chronologies of the event  Historians should attempt to minimize the number of competing sources, perhaps by eliminating the less credible ones
Once this is achieved and several sources are left, as often happens, historians may try, though on occasion unsuccessfully, to determine independently of the usual sources which date is more likely to be right
Some of the events for which there are conflicting chronologies and for which attempts have been made by historians to determine the right date cannot be dated reliably by historians.