"After Flair" is the year system used by the majority of Munkrufians.
It was first used by the Flairian group after they had started work on Flair, as they disagreed about what year it was in the old year system they had used on Earth (the "Common Era" year system.) It was adopted by Gigacavern due to the fact Gigacavern was set-up by a Flairian adventurer, and the few local people had lost track of the year.
The people of the town of SpawnVille on the continent of Spaval had managed to hang on to the CE system, by agreeing the year they had crash landed was 2545 CE (as opposed to other suggestions which varied from 2520 to 2700), but this system could not work due to other places disagreeing with their time, making it confusing to work with other towns which used a different crash landing date. This was resolved when "After Flair" was adopted in 40 AF by all the major towns/forts at the time.