‘他日’ literally combines ‘he/other’ (他) and ‘day’ (日), but functions as a fixed expression meaning ‘another day’ or ‘in the future’—often with a sense of formality, reflection, or polite deferral. It does not refer to someone else’s day, nor is it used for immediate or specific future times like ‘tomorrow’ or ‘next week’. Instead, it conveys an indefinite, often dignified or literary, point in time ahead—common in written Chinese, speeches, or courteous conversation.

The term appears frequently in classical and modern formal contexts: when making promises (‘I’ll do it another day’), expressing hope (‘We’ll meet again another day’), or reflecting on future possibilities. Though grammatically a noun, it often modifies verbs adverbially without particles (e.g., 他日必成), giving it dual syntactic behavior. Learners should avoid using it in casual speech where ‘以后’ or ‘改天’ would sound more natural.