The sky has unforeseen storms; people have misfortunes that strike from one day to the next.
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Literary, classical-feel word (lit. 'dawn-dusk') most alive in fixed expressions: 危在旦夕 'in imminent peril', 命在旦夕 'life hangs by a thread', and the proverb 天有不测风云，人有旦夕祸福 (Yuan-dynasty drama 《合同文字》). Rare on its own in conversation.
