The Book of Hours is a devotional book made popular in the later Middle Ages, and is the most common type of surviving medieval illuminated manuscript, which contains a collection of texts, prayers and psalms, for Christian devotions. The Books of Hours was traditionally written in Latin although many were written entirely or partially in European languages.

Concentrating on the archangels and it gives hourly, daily, weekly and monthly prayers, and requests directed to the archangels.

According to Peter de Abano, “…. that the first hour of the day, of every Country, and in every season whatsoever, is to be assigned to the Sun-rising, when he first appeareth arising in the horizon: and the first hour of the night is to be the thirteenth hour, from the first hour of the day”

What does this mean? The first hour occurs from Sunrise and the first hour of the night occurs 13 hours from Sunrise. To calculate the actual hours you need to know the time of the sunrise. From this time the other ours are easily calculated, using 60 minutes per hour.

Reference:
Peter de Abano: Heptameron, or Magical Elements.