It was during 1840.
David got some real zingers during the "Summer of 1970" storyline. I loved this one episode where Barnabas and Julia (who today would be said to be in full "helicopter parent mode") are discovered by David in his room with, I think, Carolyn, and he angrily blurts out: "What is this, a convention?" I sometimes wonder if he improvised some of that dialogue.
I've never read just exactly why David left. I think it was some kind of dispute with the "Master of Dark Shadows" (i find it hard to type those words without laughing).
G.