Why dates and years are different in different calendars?

Different timekeeping methods existed from the prehistoric up until the Neolithic period. The units utilized for the timekeeping means of various historical societies can be classified into the day, solar year, and lunation. Since the units used are diverse, the dates and years in different calendars are also varied.

The first discovered calendars came from the Bronze Age, which includes the Sumerian and Egyptian calendars. Then, a multitude of calendar methods from the Ancient Near East began in the Iron Age, with most derived from the calendar of Babylonia. The Persian Empire has a calendar of its own that gave birth to the emergence of the Hebrew and Zoroastrian calendars.

There were also different Hellenic calendars created in Classical Greece. Moreover, it sparked calendars outside of its umbrella, such as the Hindu calendar and the Roman calendar. The latter featured ancient vestige from the ten-month solar year of the pre-Etruscan era.

In 45 BCE, Julius Caesar revised the Roman Calendar. The new Julian calendar no longer relied on new moon observations. Instead, they systematically followed a simple algorithm. Later on, the system incorporated a leap year every four years. The said action solidified the separation of the calendar month from the lunar one.

304 days composed the old Roman year with a total of ten months, starting from March. Julius Caesar, however, thought that the system began to become unusable. With that, he made drastic changes in his third term in the consulship. The year of 709 AUC commenced in January and ran 365 days until December 31.

More improvements were effected in the Julian calendar. They added the concept of having a leap year every four years, starting in 737 AUC. The new Julian calendar has been widely used in Europe until 1582.

Marcus Terentius Varro then made the AB urbe condita epoch, presuming Rome’s foundation in 753 BCE. The said system stayed in use in the early medieval period prior to the Dionysian era was accepted in the Carolingian period.

While the seven-day week can be attributed to the Ancient Near East era, the start of usage of planetary weeks can be dated during the surge of the Roman Empire.

In 1582, the Julian calendar was refined in the form of the Gregorian calendar. Despite different calendars being utilized in different regions, cultures, and millennia, Western historical scholarships have based their dates and years from the Gregorian calendar. This means that they bypass the geographical area and the historical period the event has occurred. Instead, they follow the system of the dominant Gregorian calendar. Rarely, some historians still prefer to utilize both dates, one from the Gregorian calendar and the other from the original calendar.

Today, the Gregorian calendar, also called the Christian or Western calendar, is the most widely used calendar system in the world. Its name was derived from Pope Gregory XIII, who launched the method in 1582.

The Gregorian calendar was a refined version of the Julian calendar. The reform was said to be made to halt the calendar’s drift concerning solstices and equinoxes, especially the vernal equinox, the determining date of Easter. By reforming the Julian calendar, it allowed Easter to be celebrated at the time of the year it was originally supposed to be and as set by the Church.

Initially, the Gregorian calendar system was used by European Catholic churches. Others continued to utilize the old Julian Calendar. But, for convenience in the international trade, more countries followed suit in using the dominant Gregorian calendar.