Eras are just a convenient way to create blocks of history .. I reality no era ends .. it is a slow moving circle .. in three dimensions of time :-)
Discussion
Time is just an illusion. Past, present and future are happening all at once. We just perceive time as linear because we are still more unconscious than conscious.
Even in our linear reality we are blinded by illusion. You accurately point out that what we perceive as linear time is actually a circular upwards spiral