The Anglo-Norman term was a borrowing of Old French ure, a variant of ore, which derived from Latin hōra and Greek hṓrā (ὥρα). Like Old English tīd and stund, hṓrā was originally a vaguer word for any span of time, including seasons and years. Its Proto-Indo-European root has been reconstructed as *yeh₁- ("year, summer"), making hour distantly cognate with year. The time of day is typically expressed in English in terms of hours. Whole hours on a 12-hour clock are expressed using the contracted phrase oclock, from the older of the clock. (10 am and 10 pm are both read as "ten oclock".) Hours on a 24-hour clock ("military time") are expressed as "hundred" or "hundred hours". (1000 is read "ten hundred" or "ten hundred hours"; 10 pm would be "twenty-two hundred".) Fifteen and thirty minutes past the hour is expressed as "a quarter past" or "after" and "half past", respectively, from their fraction of the hour. Fifteen minutes before the hour may be expressed as "a quarter to", "of", "till", or "before" the hour. (9:45 may be read "nine forty-five" or "a quarter till ten".) History
Antiquity
The ancient Greeks kept time differently from the way we do today. Instead of dividing the time between one midnight and the next into 24 equal hours, they divided the time from sunrise to sunset into 12 "seasonal hours" (their actual duration depending on season), and the time from sunset to the next sunrise again in 12 "seasonal hours".
Initially, only the day was divided into 12 seasonal hours and the night into 3 or 4 night watches.By the Hellenistic period the night was also divided into 12 hours. The day-and-night (νυχθήμερον) was probably first divided into twenty-four hours by Hipparchus of Nicaea. The Greek astronomer Andronicus of Cyrrhus oversaw the construction of a horologion called the Tower of the Winds in Athens during the first century BCE. This structure tracked a 24-hour day using both sundials and mechanical hour indicators.The canonical hours were introduced to early Christianity  from Second Temple Judaism. By AD 60, the Didache recommends disciples to pray the Lords Prayer three times a day; this practice found its way into the canonical hours as well. By the second and third centuries, such Church Fathers as Clement of Alexandria, Origen, and Tertullian wrote of the practice of Morning and Evening Prayer, and of the prayers at the third, sixth and ninth hours. In the early church, during the night before every feast, a vigil was kept. The word "Vigils", at first applied to the Night Office, comes from a Latin source, namely the Vigiliae or nocturnal watches or guards of the soldiers. The night from six oclock in the evening to six oclock in the morning was divided into four watches or vigils of three hours each, the first, the second, the third, and the fourth vigil.The Horae were originally personifications of seasonal aspects of nature, not of the time of day.