Here is a hypothetical timeline of the history of the planet Eternia.  The calendar is based upon the founding of the planetary, monarchial government of Eternia, with Year 0 being the date of the planetary monarchy's foundation by King Jarod

10 billion years before Year 0:  Big Bang creates the known universe that contains Eternia, the planet Earth, and countless other worlds.  Eternia soon after forms at center of known universe.  Beginning of Preternia.

100,000 to 10,000 years before Year 0:  Humans and other races evolve and emerge on Eternia:  Skytree and the dragon Granamyr are both born during this time.

10,000 to zero years before Year 0:  war between  Various civilizations (both human and non-human) of Ancient Ones rise and fall during this time.

10,000 to 5000 years before Year 0:  First Ones and other Eternian people leave Eternia via magical space portals for a new world, the planet Etheria.

8000 to 5000 years before Year 0:  worship of the Goddess becomes the dominant faith on Eternia.

4555 to 4498 years before Year 0:  war between Eternia's humans and Dragons led by Granamyr.

3000 to 2500 years before Year 0:  ancient city depicted in the episode Keeper of the Ancient Ruins is first built, along with it's robotic guardians, and for several centuries is a leading Ancient One civilization until it's finally abandoned around five centuries after it was first built.

3000 to 100 years before Year 0:  the Ape Clan and the Snake Clan gradually become the strongest human nations on Eternia. 

40 years before Year 0:  events in the episode The Time Corridor occur.

35 years before Year 0:  birth of King Jarod, son of Zilora, female leader of the Snake Clan.

11 Years before Year 0:  Coalition of human and non-human nations under leadership of Jarod, including the Snake Clan, the Ape Clan, and remnants of Ancient One societies, wages war against alien, offworld invaders desiring to conquer Eternia.  These invaders are now assumed to be the Evil Horde, directly led by Horde Prime himself (before Hordak assumed a leadership position within the Horde).

Year 0:  Formation of the planetary government of Eternia by King Jarod.  End of Preternia according to some historians.

Year 0 to Year 2200:  the ruling monarchy of Eternia has it's highs and lows. 

Year 2179:  King Hiss assumes supreme leadership of Eternia's Snake Men.

Year 2200 to Year 2500:  Time of the Great Wars.  Much of the planet is conquered and occupied by King Hiss and his Snake Men.  The seat of the legitimate government of Eternia for several centuries is moved from Eternos to the newly built Castle Grayskull.

Year 2490:  King Grayskull assumes the throne as legitimate ruler of Eternia.  With the aid of He-Ro and other warriors, he wages war against King Hiss. 

Year 2492:  Hordak and the Evil Horde invade Eternia. 

Year 2492 to Year 2498:  King Grayskull, the Evil Horde, and King Hiss wage three-way war against each other.

Year 2498:  temporary two-year truce between three factions.  Creation of the Three Towers to signify the truce.

Year 2500:  The Evil Horde and Snake Men team up in futile attempt to topple King Grayskull's regime.  Grayskull mortally wounded by Hordak.  The Horde and King Hiss are banished from Eternia.  End of Preternia and beginning of modern Eternian era according to some historians.  Council of Elders assumes advisory role for Eternian monarchs descended from King Grayskull and his wife Queen Veena.  Veena becomes first Sorceress of Castle Grayskull.  Seat of Eternian planetary monarchy moves back to Eternos.

Year 2667:  Council of the Wise gives the Golden Disks of Knowledge to the government of Eternia for safekeeping. 

Year 2920:  Birth of King Miro.

Year 2945:  Miro assumes throne of Eternia.

Year 2952:  Birth of Keldor, son of Miro.

Year 2954:  Birth of second son of Miro and younger brother of Keldor, the future King Randor

Year 2968:  Zanthor, on the advice of the Council of Elders, is appointed the guardian of the Golden Disks of Knowledge by King Miro.

Year 2970:  King Miro disappears.  Council of Elders proclaims Randor the legitimate ruler of Eternia, declaring Keldor to be too evil.

Year 2971 to Year 2973:  Keldor (now transformed by magic into Skeletor) wages war on Randor's royal government with the aid of Hordak and the now-returned Evil Horde.  During this two-year period, Teela Na replaces Kuduk Ungol as Sorceress of Castle Grayskull and defeats Morgoth, a temporary and uneasy ally of the Skeletor-Horde coalition.  Skeletor, Hordak, and the Evil Horde are defeated.  Council of Elders disappears from Eternia.

Year 2975:  Skeletor forms band of Evil Warriors and occupies Snake Mountain.  Teela Na gives birth to daughter TeelaQueen Marlena arrives on Eternia in spaceship from Earth

Year 2976:  Teela Na's husband dies fighting Skeletor's Evil Warriors.  Duncan becomes Man-at-Arms.  Marlena marries King Randor.

Year 2977:  Birth of Prince Adam and Princess Adora.  Soon after, Hordak temporarily returns to Eternia and kidnaps Adora in revenge for his earlier defeat by Randor's regime.

Year 2980:  Darkdream menaces much of Eternia.

Year 2984:  Prince Adam finds his pet tiger Cringer.

Year 2985:  Orko first arrives on Eternia from Trolla.

Year 2991:  Adam first given Sword of Power.  Adam and Cringer first become He-Man and Battle Cat.

Year 2995:  formation of Heroic Warriors to combat Skeletor and the Evil Warriors.

Year 2996 to Year 2998: events that appear in He-Man and the Masters of the Universe (classic cartoon) take place.

Year 2999:  Prince Adam discovers his lost twin sister Adora on Etheria, the supposedly twin planet of Eternia.

Year 3000: present day.

Year 3244 (possible future):  The Tri-Solar System discovered by space explorers from Eternia.  The planet Primus within the Tri-Solar System soon becomes the target of colonization by people from Eternia.

Year 4000 (possible future):  events in The New Adventures of He-Man occurs.