What would be a good, culturally neutral year 0? The Western/Christian/common era calendar starts with the birth of Christ. Other calendars have used similar cultural milestones, IE the founding of Rome, the supposed age of the world according to the bible (Byzantine calendar), the enlightenment of the Buddha, etc. 

So what would be some events that are so univerally important that it would make sense for the entire world to acknowledge them as 'year 0'? What would be a culturally neutral starting-point for a pan-human calendar?