The Julian Calendar
The Romans used 753 BCE, the traditional year of the Founding of Rome, as a temporal zero, but that was a convenient fiction. Unlike the Greeks, who started their hours at sunrise, the Romans started their day of 24 hours at midnight. However, despite the availability of regular water based clocks known as clepsydra, from the middle of the second century BCE, the Roman hora, or hour, was variable: daylight was divided into 12 hora, as was night. Accurate timekeeping was important only in special situations such as law courts, where the amount of time a speaker could have was regulated.
The Roman preoccupation with time has left us an enduring legacy of Roman calendars. Before the reforms of Julius Caesar, the Roman calendar year was drifting away from the solar year, requiring the insertion of an intercalary month to close the gap. Caesar addressed the problem on the advice of astronomers, resulting in the Julian calendar, consisting of 12 months, each with its own fixed number of days apart from February. There is a useful rule based on the knuckles of the human hand for remembering the number of days in each month. Starting with January on the outer knuckle of the left hand, we associate February with the space between the next knuckle, March is associated with the next second knuckle in, and so on. July is associated with the innermost (fourth) knuckle of the left and then August is associated with the innermost knuckle of the right hand, and so on. For each knuckle month we associate 31 days, whilst for each space month we have 30, apart from February, which has 28 normally and 29 on a leap year.