The word 改天 is formed by combining two characters: 改 (gǎi), meaning "to change" or "to alter", and 天 (tiān), meaning "day" or "sky". It is a conceptual metaphor where the action of "changing" is applied to the abstract notion of a "day", creating the figurative meaning of moving a planned event from one specific day to another, unspecified day in the future. This formation directly visualizes the idea of rescheduling by conceptually swapping one day for another.