***Spoiler Alert - Do not read before you get to chapter 29***
An epoch, for the purposes of chronology and periodization, is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided by congruity, or by following conventions understood from the epoch in question. The epoch moment or date is usually defined from a specific, clear event of change, an epoch event. In a more gradual change, a deciding moment is chosen when the epoch criterion was reached.
-Wikipedia
Our intrepid group just had a moment that redefines their lives from that point on; it creates a new beginning and relegates everything before that moment to simply memories.
Everything about them will now be defined from that moment in time.
The scope of an epoch event is relational to the individual affected; the birth of my first child was a defining moment for me. There was life before kids and then life with kids. That epoch moment was more impactful to me than anything short of the invention of rolled toilet paper.
This event created a definitive "Before" and "After" in the characters lives, they would now time all future events to the Epoch event itself. Two weeks later or ten years later regardless it all would measure back to that Epoch event.
-Emmeran