The timeline of Undertale Rho has been broken up into story arcs for better understanding & convenience.
Major Story Arcs
Pre-History Arc: Unknown - 2134 BC
The Pre-History Arc covers many, many important areas of history. Being the longest arc, we see events ranging from the Monsters origins to the Anendotos Kingdom's ancestral founding and origins.
Dark History Arc: 505 BC - 1125 AD
The Dark History Arc is one of the longest story arcs, documenting the backstory of [REDACTED] and eventually morphs into the history as to how all the Monsters went from world wide to just being located in the kingdom of Anendotos. The protagonist of this arc is [REDACTED]. The antagonists of this arc are multiple humans over the years.
War Arc: 1256 - 1258
The War Arc is the story arc that documents the history of the war between the Humans and the Monsters. The protagonists of this arc are King Asgore and Dr. W.D.Gaster. The antagonists of this arc are King [redacted], Lord [redacted] and the six mages, [redacted.]
Family Arc: 1407 - 1410
The Family Arc documents the history of a few days before Chara fell into the Underground to when Asriel dies. The protagonists of this arc are Chrysanthe Charaktiras, Prince Asriel, King Asgore, and Queen Toriel. There are no real antagonists in this arc.
Fallen Arc: 1487 - 1873
The Fallen Arc is the time from when the second human, [redacted], fell into the underground to when the last human (besides Frisk) fell into the Underground. The protagonists are the fallen humans. The antagonists are the monsters.
Laboratory Arc: 1961 - 1976
The Laboratory Arc tells the backstory of Sans Gaster, Papyrus Gaster, and The Shadowy Figure. The protagonist/antagonist system doesn't exactly work here as they are all protagonists and antagonists at the same time. This story arc technically goes until 1990 which is when The Shadowy Figure kills Wing.
Madman Arc: 2010 - 2015
The Madman Arc takes place on the surface, focusing on someone who has the ability to reset. The final human, Frisk, remembers every reset. While he tells people that time keeps resetting, no one believes him, causing him to be the target of numerous people accusing him of being crazy. Eventually, the madman loses the ability to reset and dies soon after.
Flowey's Arc: Early September 2015
Flowey's Arc is the 15 days that Flowey keeps replaying over and over. He eventually loses the ability in the morning of September 15th, 2015.
Underearth Arc: September 15th, 2015 - September 16th, 2015
The Underearth Arc is Undertale for the first half, though deviates from it during the latter half. It lasts from the evening of 9/15/15 and goes until the morning of 9/16/15. The Underearth Arc takes place within Mount Ebott.
TNR Terrorist Group Arc: December 22nd, 2015 - September 7th, 2025
The TNR Terrorist Group is a group of humans and monsters that use extreme measures to prevent another human/monster war from breaking out. The group is destroyed when a raid on their HQ proves failed until Frisk goes in, and exits with the leader of the group in custody. The TNR era ends when he is executed a few days after capture.
Surfaced Arc: September 15th, 2025 - September 25th, 2025
Onyx Extinction Event: February 13th, 2026 - September 11th, 2026
Dark Scare Arc: 2030 - 2031
Eternal Consumption Arc: 2035 (Real beginning date unknown)
Fire Rain Arc: 2055
Corbel Extinction Event: 2075
Minor Story Arcs
The Exodus Arc: 1408
The Exodus Arc is when the monsters had migrated from The City of Home to go and found The City of New Home.