Why can’t any particular event be considered the beginning of a new time?

The event itself is a very short period of history, an incident. It cannot immediately become the beginning of a new era, but it can cause consequences that would change history. Take the Reformation for example. This is not one event, not two, or even ten. This is a whole trend that does not depend on one incident. During that time, it is impossible to name one specific uprising that led to a split in the church. History changes only from a chain of different events. The tendency entails them, forming a historical process. And only after that people realize that they are on the threshold of a new time.