my_torah

By Googling I have found the following:

 

Ancient cultures had a variety of ways they defined the beginning of a day:

• Evening-to-evening: The Babylonians, Jews, and (most) ancient Greeks used this system. 
• In ancient times in Egypt, Israel, Mesopotamia, Greece and Rome the night was divided into 3 or 4 “watches”. 
• Dawn-based: Egyptians, Hindus and early Romans counted the day from sunrise.
• Midnight: In ancient China the day began at midnight. Maybe it’s something to do with running a big empire, because the later Romans were the forerunners of our current midnight-to-midnight system which evolved in the Roman Empire some time after 27BCE. References to night watches ("vigiliae") dividing the night into three or four parts appear in Roman writings from the Republic period (before 27 BCE). After that, references to the Roman day starting at midnight become more frequent. 

 

The Zoroastrian day (attested to in the 9th century) began at dawn. The Zoroastrian calendar adopted Egyptian, Babylonian and Roman practices. They also had “watches” as divisions of the night. 

Islam adopted the Jewish system of evening-to-evening days. 

The Western Christian Church has a mix. They retain the Jewish evening-to-evening day for religious festivals like Christmas Eve, but for the liturgical calendar and canon law the midnight-to-midnight definition is used.

In the Orthodox Church the liturgical day starts in the evening.  

 

Conclusion:

 

At the times when our Mishnah and Gemarah were written down there would have been a variety of competing ideas about when a day began, from evening-to-evening, dawn-to-dawn,  or midnight-to-midnight. 

 

The night would have been measured as divided into 3 or 4 equal watches or into 12 equal hours.