Because the term "yomi (to read)," which means counting cycles of the moon phase, was spelled "余美" ("yomi") or "餘美" ("yomi") in Chinese characters in order to reflect the Japanese pronunciation and both were spelled "ヨ乙・ミ甲" ("yo・mi") in the Jodai special kana orthography-which conforms precisely to the spelling of "yomi" of "Tsukuyomi"--it would be safe to conclude that the name Tsukuyomi came from counting days and months.
上代特殊仮名遣では、「暦や月齢を数える」ことを意味する「読み」の訓字例「余美・餘美」がいずれもヨ乙類・ミ甲類で「月読」と一致していることから、ツクヨミの原義は、日月を数える「読み」から来たものと考えられる。