(Internet slang, especially anime and manga fandom) events or things from the past which one wants to whitewash/erase from history (due to it being embarrassing, poorly received, etc.); dark past

study of history

historic, historical / already-happened, past / former, old, traditional

Chinese calendar

Communist Party of China

sinicization

China, as opposed to Japan and other islands / mainland China

China proper

the Chūgoku region (western part of Honshu comprising the prefectures of Okayama, Hiroshima, Yamaguchi, Tottori, and Shimane)

Chinese (any language spoken in China) / Chinese (class of languages) / Chinese (Mandarin)