1The passage of time — years and days going by; often used to talk about one's life, or how much time something has taken. Slightly more literary than 時間 or 年月.
月日が経つのは本当に早い。
How time flies.
二人が出会ってから、もう十年の月日が流れた。
Ten years have passed since the two of them met.
苦しい月日を乗り越えて、彼は今の地位を築いた。
He built his current standing by overcoming difficult years.
月日が解決してくれることもある。
Sometimes, time itself resolves things.
2The moon and the sun, taken together — an old-fashioned, literary usage found in classical writing and waka poetry, rarely used this way in everyday modern Japanese.
古典の和歌には、月日を天地の運行の象徴として詠んだものがある。
Some classical waka poems depict the moon and sun as symbols of the movement of heaven and earth.