The Roman day was segmented into 24 hours with 12 hours of daylight and 12 hours of darkness. There was particular emphasis placed on the 3rd, 6th, and 9th hours as these divided the sky (and path of the sun) into four distinct segments of daylight. This practice of splitting the day into two twelve hour halves is presumed to have been adopted from the Egyptians. The actual device used for timekeeping was known as an anaphoric water clock.

The sexagesimal (Base 60) time system that we are familiar with, and that was used by the Romans, is though to have arisen from Mesopotamia or Egypt. The prevalence of 12 may have arisen from the ability to count to twelve on hand by pointing to each finger bone with one's thumb. 60 is turn made up of 5 groups of twelve. Bearing all this in mind gives you 60 seconds to a minute, 60 minutes to an hour, and 24 hours to a day (with two twelve hour halves.)

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Sources

• Birth, K. K. (2014). The Vindolanda Timepiece: Time and Calendar Reckoning in Roman Britain. Oxford Journal Of Archaeology, 33(4), 395-411. doi:10.1111/ojoa.12044

• Christensen, B. (1998). Time's Pendulum: The Quest to Capture Time - from Sundials to Atomic Clocks. Booklist, (18). 1575.

• Macey, Samuel L. (1989). The Dynamics of Progress: Time, Method, and Measure. Atlanta, Georgia: University of Georgia Press. p. 92. ISBN 978-0-8203-3796-8.