How did the ancient Romans calculate time?

The Holy Roman Empire is one of the most popular and powerful empires in history. It stretched from present-day Egypt, Algeria, and Morocco all the way to the Danube River, including a large part of present-day England. The western part of the Roman Empire existed from 27 BCE until its fall in 476 CE, while the eastern part continued to exist until 1452.

It’s easy to conclude that the ancient Romans did not use the clocks that modern people use today, nor did they use the Gregorian calendar, which came into use only in 1582. Instead, the Romans had their own ways of keeping track of time, which we will explain in this article.

Short-term time measurement – day and night hours

Day hours

The Romans divided the day into 12 hours. This referred only to the period during daylight, from sunrise to sunset. The night period had a different division, which we will discuss later. The first hour, or Hora Prima, began at sunrise, and the last, Duodecima Hora, ended at sunset.
Below is a list of all 12 day hours in Ancient Rome:

Prima Hora (First Hour): It began at sunrise, and this time was considered the start of the day.
Secunda Hora (Second Hour): Romans saw this as the next first hour.
Tertia Hora (Third Hour): Around mid-morning. For the Romans, this was approximately three hours after sunrise.
Quarta Hora (Fourth Hour): It continued after mid-morning.
Quinta Hora (Fifth Hour): Late morning, just before noon.
Sexta Hora (Sixth Hour): Noon. This is where the term “siesta” (afternoon rest) comes from, as people often took breaks at this time.
Septima Hora (Seventh Hour): The first hour after noon, early afternoon.
Octava Hora (Eighth Hour): Mid-afternoon.
Nona Hora (Ninth Hour): Late afternoon, roughly around 3 PM.
Decima Hora (Tenth Hour): Late afternoon, closer to sunset.
Undecima Hora (Eleventh Hour): Approaching sunset, the last part of daylight.
Duodecima Hora (Twelfth Hour): The last hour of daylight, ending at sunset.

These hours were not always of the same length. Their duration depended on the season. In summer, due to the longer day, one hour would last up to 75 minutes, while in winter, it would last about 45 minutes.

Sundial
To accurately track the hours during the day, the Romans used a sundial, which they called Solarium. This word comes from the Latin word Sol, meaning Sun.
The sundial worked on a simple principle that tracked the position of the sun and consisted of two parts:

Gnomon – The upright part whose purpose was to cast a shadow that indicated the time on the plate below.
Plate, or Dial Plate – A flat plate marked with 12 hours. As the sun moved across the sky, the shadow created by the gnomon would shift, pointing to a specific hour on the plate.

The sundial’s shape and design were not always the same. They could vary depending on location and the angle of the Sun at different times of the year.

Night hours
Night in ancient Rome was divided into four segments called Vigilae. This word translates to “watch” because they were primarily used for military purposes. Each watch lasted around 3 hours, but, like the day hours, their duration varied depending on the season. Below is a list of the night watches.

Prima Vigilia: The first watch. From sunset to around 9:00 PM.
Secunda Vigilia: The second watch. From 9:00 PM to midnight.
Tertia Vigilia: The third watch. From midnight to around 3:00 AM.
Quarta Vigilia: The fourth watch. From 3:00 AM to sunrise.

Night hours were measured less precisely than day hours due to the lack of sunlight, so the Romans often relied on general estimates rather than exact measurements during the night.

Long-term time measurement
The people of ancient Rome had multiple systems for tracking years, often synchronizing them with each other. One of these systems was the consular system, where years were named after the consuls elected in that year. Each year, two consuls were elected, so if someone said, “During the consulship of Cicero and Antonius,” they were referring to the year 63 BCE by our current calendar.

Another way the Romans referenced years was by imperial reigns. This system was mostly used for official documents. For example, when a document would say, “In the seventh year of Emperor Augustus,” who reigned from 27 BCE to 14 CE, it would refer to the year 20 BCE by our reckoning.

In addition to these systems, the Romans had two other concepts for periods of 5 and 100 years. A 5-year period was called Lustrum, and a 100-year period was called Saeculum.

Lustrum was traditionally associated with a purification ritual, whose name came from the word Lustratio, meaning purification. At the end of every 5-year period, high magistrates or censors performed the Lustrum ritual. This ritual involved making sacrifices to the gods to ensure their favor for the Romans and to purify the people, but it also related to social and political matters such as tax assessments, population censuses, and monitoring public morality.
Saeculum was much longer, referring to a period of about 100 years, or roughly the length of a human life. For the Romans, these periods served as cultural and historical markers through which they observed changes and evolution in their society and civilization. Important historical events, such as the founding of Rome or significant political and social changes, were referred to in relation to the Saeculum to emphasize their impact and importance.

Calendar
In addition to the systems we’ve mentioned, the Romans also used a calendar. While they had various systems for tracking years and periods of time, the Romans also counted years linearly, just like we do today.

Their year-counting began with the founding of the city of Rome, which they believed occurred in 753 BCE. This system was called Ab Urbe Condita (AUC), meaning “from the founding of the city.”

The first calendar used by the Romans is known today as the Early Roman Calendar. It was a lunar calendar of 10 months and 304 days, beginning with March. Later, two more months (January and February) were added, making the year 355 days. This calendar required regular adjustments because it fell out of sync with the seasons.

Below is a list of the original 10 months of the Early Roman Calendar:

Martius (March)
Aprilis (April)
Maius (May)
Iunius (June)
Quintilis (Later renamed Julius after Julius Caesar)
Sextilis (Later renamed Augustus in honor of Emperor Augustus)
September (7th month)
October (8th month)
November (9th month)
December (10th month)

Caesar’s reform
The calendar we use today is the “Gregorian Calendar,” which is used worldwide. It was introduced by Pope Gregory XIII in 1582. This calendar is based on the “Julian Calendar,” which came from the reform of the Early Roman Calendar. The reform was made by the Roman general and statesman Gaius Julius Caesar in 45 BCE.

The Julian Calendar had 12 months and 365.25 days. By adding a leap year every four years, the extra quarter of a day was compensated for.

Below is a list of the 12 months of the Julian Calendar (the reformed Early Roman Calendar):

Ianuarius (January) – 31 days
Februarius (February) – 28/29 days (Leap years)
Martius (March) – 31 days
Aprilis (April) – 30 days
Maius (May) – 31 days
Iunius (June) – 30 days
Iulius (July) – 31 days
Augustus (August) – 31 days
September – 30 days
October – 31 days
November – 30 days
December – 31 days

Days in a month
The Romans did not count the days in a month linearly like we do today. Instead, they counted backwards from three fixed points in each month. These three points were called:

Kalendae: The first day of the month.
Nonae: Usually the 5th day, but the 7th in March, May, July, and October.
Idus: Usually the 13th day, but the 15th in March, May, July, and October.

For example: