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Date and Time

The Calendar

The calendar used in Pompeii at the time of the Vesuvian eruption was the Julian calendar, a reform of the original Roman calendar which was claimed by Roman writers to have been invented by Romulus, the founder of Rome in around 753BC. The original calendar was based on the cycles of the moon. As the time between new moons averages 29.5 days, its months were constructed to be either hollow (29 days) or full (30 days). The calendar covered 304 days and was composed of 10 months, starting from March (Martius) through to December.(The later months were named based on their position in the calendar: e.g. October from octo (8), November from novem (9), etc.

Numa Pompilius, the second of the seven traditional kings of Rome, reformed this calendar by adding January and February (with 29 and 28 days respectively) to the original ten months in around 713BC, at the same time removing a day from 6 of the months to create a calendar of 355 days (an example is pictured right, showing January to May). To keep the new calendar aligned with the solar year an occasional leap month was added mid February.

In the Roman Republic, the years were not numbered, but were named after the consuls in power at the beginning of the year. Later, historians began to number years from the founding of the city of Rome (
ab urbe condita - abbreviated to a.u.c.).

While Julius Caesar was Pontifex Maximus the calendar was again revised, with the result being the Julian calendar. The Julian calendar, like today's Gregorian calendar, had a regular year of 365 days divided into 12 months, with a leap day added to February every four years.

In addition to modifying the structure of the calendar, the month Quintilis was renamed Iulius (July) in honour of Julius Caesar in 44 BC and Sextilis was renamed Augustus (August) in honour of Augustus in 8 BC.

Name of Month Derivation
Ianuarius After Janus, the god of the doorway.
After the purification ritual Februa held on the 15th of February.
Martius After Mars, the god of war.
Aprilis Possibly from the Latin aperire, to open.
Maius After the Greek goddess Maia.
Iunius After the goddess Juno, wife of Jupiter.
Iulius (Quintilis)
After Julius Caesar. Previously called Quintilis, the fifth month.
Augustus (Sextilis)
After Augustus. Previously called Sextilis, the sixth month.
Septembris The seventh month.
Octobris The eighth month.
Novembris The ninth month.
Decembris The tenth month.
Table 1: The months of the year.

Numbering the Days

Unlike dates today, which are numbered sequentially from the beginning of the month, the Romans counted backwards from three key days: the
Kalends, the Nones and the Ides. These key days were as follows:

Key Day
 Kalends (Kalendae)  The first day of each month. .
 Nones (Nonae)  The ninth day before the Ides (i.e. the 5th or 7th day of the month). .
 Ides (Idus)  The 13th day of each month (15th for March, May, July and October). .
Table 2: The Key Days.
The soothsayer's warning to Julius Caesar, 'Beware the Ides of March,' referred to the 15th of March (
Idus Martiae). The day preceding a key day was known as 'on the day before' (pridie). Thus the 14th of March was pridie Idus Martias. The remaining days were identified by counting backwards (inclusively) from the Kalends, Nones, or the Ides. For example, the 3rd of March would be ante diem V Nonas Martias - 5 days before the Nones. As in all written things the Romans abbreviated dates. Thus the above date would generally be written a.d. V Non. Mart.


Time was not an exact thing in ancient Rome. Hours were originally calculated for the time
between the rising and the setting of the sun. This interval was divided into 12 equal hours. Conversely, night hours were divided between sunset and sunrise. Instead of being of fixed duration, however, these intervals inevitably changed throughout the year. On any given day, the duration of the day hours was different from the length of the night hours.

At the winter solstice, when the day had only 8 hours and 54 minutes of sunlight against a night of 15 hours and six minutes, the day hour was only a little over to 44 minutes long while the night hour lengthened to one hour and 15 minutes. At the summer solstice the position was exactly reversed; the night hour was at its shortest while the day hour reached its maximum length.

The winter solstice day hours were:

Hour (Hora)
From To
10:31 11:15
11:15 12:00
12:00 12:44
12:44 1:29
1:29 2:13
2:13 2:58
2:58 3:42
3:42 4:27
Table 3: Winter Solstice: the hours of the day.

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