at the bottom of the hour / at the top of the hour

at the bottom of the hour =  no meio de cada hora (1:30, 2:30, etc.)We’ll have  new headlines at the bottom of the hour.

at the top of the hour

(às tantas horas em ponto, na hora exata: 1 hr, 2 horas etc, no começo de cada hora)

At the top of the hour,. the reporter always announced some news.  =  A cada hora exata, o repórter…

at the top of the hour           at the bottom of the hour

Pequeno Dicionário de Dicas e Dificuldades