at the top of the hour
At the start of the hour (such as 12:00, 3:00, etc.). And we'll cover that news story at the top of the hour.
at the top of the hour
Fig. at the exact beginning of an hour. (Alludes to the big hand on a clock pointing to the 12. Often heard on television or the radio. See also at the bottom of the hour.) Every class in my school starts at the top of the hour. Our next newscast will be at the top of the hour.