Where does “月日” come from?

月日 (Old Japanese) comes from Old Japanese 月, from Proto-Japonic tukuy, from Proto-Japonic tuku.

月日 (Old Japanese): the moon and sun

Definitions

  1. the moon and sun

Ancestry of “月日”, step by step

月日 traces back through two separate lines of ancestry.

via Old Japanese 月

StepLanguageWordMeaning
1Old Japanese月the moon; a month; a menstrual cycle,...
2Proto-Japonictukuymoon
3Proto-Japonictuku—

via Old Japanese 日

StepLanguageWordMeaning
1Old Japanese日the sun; sunlight, sunshine; daytime, daylight
2Proto-Japonic*piday, sun
Every word from Proto-Japonic tuku →Every word from Proto-Japonic tukuy →Every word from Old Japanese 月 →
月日 — Etymology, Origin & Word History | Etymology Explorer