late at night; at a late hour
Polite or formal expression for late-night timing, often used in business or customer-service contexts.
夜分遅くに申し訳ございません。
I apologize for calling at this late hour.
夜分遅くまでお仕事お疲れさまです。
Thank you for your hard work until late at night.
Compound of 夜分 (nighttime, evening hours) and 遅く (late). The exact historical derivation is uncertain, but the phrase is conventionally used in formal settings.