The oldest dated khachkars (cross-stone) goes back to 866 (Artsakh, Vaghuhas village in the cemetery of “Eghsu Ktor” chapel).
Year 866/I — Sargis/ erected this khachkar/to worship/remember me in your prayers.

The second oldest dated khachkar belongs to 876 in Ararat region, village Hortun (now it is in the town of Vedi).
ՅԻԵ (876):

You can find information and photos of the first khachkar in the album “Hayastan” (Yerevan 2014, p. 435), about the second one in “Ararat” regional newspaper (1986, N 151, as well as “Ejmiatsin” journal, 1987, N E-Z).
Thus, the khachkar, known also as Queen Katramide’s cross-stone, which is situated in the village of Garni is currently the 3rd one among dated khachkars belonging to 879.
From Samvel Karapetyan’s Facebook post.