It's just a Wikipedia mock-up. You find an article, edit it and then "preview" the changes rather than post them. You take a screencap of the resulting Wiki page, then cancel the edit. They're a fairly popular game on the Alternate History Dot Com forum, which is where I learned how to make them.
The dating system is just something I made up on the fly, as there's no clear calender system given in Avatar.
The "Korra Reign" is an idea that global historical periods are by Avatar. The "second Horse Year" is that, counting up the Chinese zodiac years from an Avatar's birth, you call a given year by the number of times the Avatar's life has cycled through the zodiac. Korra was born in the Year of the Ox, so by the time of Season 1 she's in her Second Horse Year.
The less romantic and more practical way of dating things is the "AO" calender: "After Omashu." It counts up from the supposed historical date of Omashu's founding, which I put waaaay in the past since, counting by the Avatar statues shown in Episode 3 of AtLA, the Avatar Universe is crazy old. The world uses Omashu as a reference point mostly due to inertia and cultural hegemony. It's the oldest city around, had a lot of pull over the ages, and the Earth Kingdom (like China) is big enough that the countries surrounding it tend to loosely orbit it in a cultural sense.
Here's a second one I had done way back when.