Avatar: The Last Airbender created a unique and fascinating world that The Legend of Korra then continued, pressing the timeline of the series not only into the future but revealing even more about its past. Fans finally had the chance to see the days of the very first Avatar, Wan, and combined with various statements throughout the franchise, gradually formed a coherent timeline of events in the Avatar world.
However, that timeline may not be as clear as it seems. Conflicting statements, logic, calendars and the use of hyperbole stretch the accuracy of Avatar's history, shrouding it in mystery that proves even more intriguing.
The world of Avatar, like many fantasy series, invokes a sense of history and depth that informs every moment of the series. Part of what makes the series so special is its unique ability to constantly hint at that depth without ever giving too much away; however, fandoms being what they are, it is inevitable that clues and hints would be put together to try to form a coherent timeline. The franchise provides plenty of fodder for such theories, but the fans who put those clues together inevitably run into difficulties.
One of the biggest moments invoking the far-reaching history of the world came in the first season episode of the original series "The Deserter," where Avatar Roku appeared to Jeong Jeong in a vision and stated that the Avatar "mastered the elements a thousand times in a thousand lifetimes." Given that known Avatars like Kuruk and Kyoshi were known to range in age from their thirties to over two hundred years old, multiplying those decades by a thousand would make the history of the Avatar quite ancient indeed. Yet this isn't the only piece of information fans have to go off.
The Legend of Korra presented the lore of Avatar Wan and the cosmic event of Harmonic Convergence, both said to occur 10,000 years before the Era of Korra. The conflict with Roku's statement would indicate that the Avatar may have been exaggerating to Jeong Jeong, but he wouldn't be the only one. When Aang and his friends met the spirit Wan Shi Tong, he introduced himself as "He Who Knows 10,000 Things." The 10,000 figure is often used in mythology much like the Chinese lore the series invokes -- more as a general statement asserting large quantities rather than a literal number -- so it would seem that none of these statements are actually definable moments in time.
That would certainly flow with the ambiguity surrounding time-telling throughout the series and the multiple calendars used periodically. Wan Shi Tong's own library contained a planetary calendar room whose markings denoted names for different eras, and the Kyoshi novels detailed an archaic calendar based around the Avatar Cycle that counts the days into the era of each Avatar. However, few utilize the Avatar calendar, and Wan Shi Tong's system was decipherable to Sokka without actually seeing use outside the calendar room. Further discrepancies between whether or not to use the Chinese zodiac, the Chinese Lunar and Solar calendars or other systems of time further obfuscate the exactitude fans can use to pin down dates.
In the end, what fans are left with is a knotted mess of vague statements, conflicting systems of measurement and rewritten histories in-universe. Even within the same episode sometimes there would be seeming conflicts, such as Zuko saying the Sun Warriors died off "thousands" of years ago before learning they only retreated from public view little more than a century prior during Sozin's reign. The characters who provide so much of the information fans feast on frequently prove hyperbolic, mistaken or even outright unreliable.
And that's why it's perfect. Real-world history and myth-making involve exactly such contradictions. Even with all the information in the world available, historians struggle to pin down or agree on precise dates. In reflecting that inexactitude, Avatar reflects realism while invoking a deeper sense of mystery and mythology that lends it more magic than it could ever have otherwise.