Game concept
End Date
From the game's encyclopedia
1453 AD is the End Date, and upon reaching it the game will end.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
End Date is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "1453 AD is the End Date, and upon reaching it the game will end." Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
for a guide, post or blog
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.