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time; years; days; (one's) life
the Moon and the Sun
「月日」 has another reading with different meanings.
月日が経つのは本当に早いですね。
Time really passes quickly, doesn't it?
あれからもう十年の月日が流れました。
Ten years have passed since then.
月日を重ねるごとに技術が上達した。
My skills improved as time went by.