AD
Anno Domini, Common Era
noun
What does the narrative origin have coming after it? Well, the way we've counted years for the past couple of millenia has a specific narrative at its origin, and everything that comes after it is AD, or anno Domini. You know which narrative we're talking about, right?
(Just in case you don't: it's the narrative of Jesus's birth, which is the year that we start calling dates A.D.)
きげんご
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
紀元後64年にローマで大火事が発生したのを知っていますか？
Did you know that in 64 A.D. there was a huge fire in Rome?
パピルスは紀元後10世紀ぐらいまで使われていた。
Papyrus was in use until about the 10th century A.D.
え、こないだ会った時より日本語めっちゃ上手になってない？紀元前と紀元後くらいちがうじゃん。
Oh, your Japanese so much better than last time we met, isn't it? It's like two different eras.