noun, noun which may take the genitive case particle 'no', adverb
every hour on the hour
Used in schedules, announcements, and formal contexts to indicate something occurs exactly at the start of each hour (e.g., 9:00, 10:00, 11:00). Often appears as 毎正時に.
The bus departs every hour on the hour.
We bring you the news at the top of every hour.
Compound of 毎 (every) + 正時 (exact hour, on the hour). The term is straightforward and likely a modern formation used in transportation and broadcasting.