What is the definition of 21 8?

"21 8" is like a time machine, but for people who don't want to deal with the 21st century's chaos. It's the time of digital dinners and AI roommates. Now, here's an unsettling example: "With 21 8, Sarah decided to schedule a therapy session with her 21-year-old self, only to find out it was a group therapy for existential dread and the latest conspiracy theory."