The Christian BC/AD calendar is just one amongst many possible systems of year counting. Various other alternative calendars exist, as we can see from the appropriate section of the FAQ, Other Dating Systems under Calendars, if your browser doesn't automatically take you there.

Indeed, BC/AD is simply a regnal calendar that refuses to change eras like normal regnal calendars. Multiple other calendar systems remain in use today. Japan still uses a regnal calendar for official purposes.

Posted on this day, Saturday 07 August; Décadi 20 Thermidor, An CCXXIX; 15 Mordad of 1400 Solar Hijri; Reiwa 3; 7529 Anno Mundi; the fourth year of the 699th Olympiad; the fifth year of Rodrigo Duterte being President.

The BC/AD system was first used, in a highly tentative fashion, by the Byzantine monk Dionysius Exiguus, who used these dates in an Eastern table (that is tracking the varying dates of Easter by year, up to his own, AD 525). He did not apply the dating to secular events, which would in the Eastern Roman Empire still be dated either by consular year (by now the consuls in Constantinople) or in the regnal dates of the East Roman Emperor.

The first significant use of AD dates for historical events was the British monk Bede in the 730s, perhaps in part because of his remove from the Byzantine Empire.

There were various dating systems in antiquity: Civic dating systems were often based on the eponymous (giving name to the year) magistrate of a city. In Athens, it was the eponymous Archon, in Rome the consuls. Thus the Romans considered 59 BC the year in which Caesar and Bibulus were consuls (Caesare et Bibulo consulibus).

The Greeks and Romans also used bench-mark dating systems, in part to sync up various civic dates, the Olympiad system which divided history into four year cycles dating back to 776 BC, and the Roman AUC, which dated events from the foundation in Rome (ab urbe condita), generally calculated by the Romans to 753 BC, although a few alternative dates existed.

In monarchic societies, including the Roman Empire, there was a tendency to describe the year based on the current monarch's regnal date; in Rome it was common to cite the number of years the reigning emperor had held tribunician power.

A highly unusual dating system---in many ways the precursor to the AD system, was employed by the Seleucid Empire, and has been recently explored in an excellent book by Paul Kosmin, Time and its Adversaries in the Seleucid Empire. Basically, the Seleucids, the successor dynasty of Alexander the Great that occupied much of the Near East, set up a dating system based on when their founder, Seleucus I, reestablished himself in Babylon in 312 BC. Events were then dated in the Seleucid Era going forward. The dating did not cycle back when there was a new king (so no year 1 of Antiochus I, Seleucus' son and successor), but rather the dynasty itself was rooted in a temporal system that could only go forward, giving it substantial, if idiosyncratic, ideological weight (just as the Christian system suggests that Christs birth has permanently created a new era of human time).

Okay. First, let’s talk about “years” which are somewhat arbitrary and variable from culture to culture. In the Gregorian calendar, a “year” is 365 days, except every fourth year, when an extra day is added to make it 366. Unless that fourth year is divisible by 100, in which case it’s 365 still, EXCEPT years that are also divisible by 400 (such as the year 2000) in which case they’re 366 days long after all. This method of measuring years is under 500 years old and didn’t spread to most of the world until European colonialism and a Eurocentric globalized economic system led to adoption of the Gregorian system as the “common” calendar for global trade.

An Islamic year, by contrast, is roughly 11 days shorter, either 354 or 355 days long. Which means that while only 1399 Gregorian years have passed since Muhammad’s arrival with his followers in Medina, the current Islamic year is 1442 (1443 will begin in two days, or Monday, August 9th by Gregorian reckoning).

Throughout human history, there have been multiple other “annual” cycles which lead to slightly different year lengths, but in nearly all the length of a year isn’t actually constant, and varies either in length by one day, or in rare cases by up to 90 days at times. This is because most calendars through human history have aimed at squaring the rotation of the earth on its axis (a day) with the period of the moons rotation around the earth (which produces tides and the phases of the moon), the period of the earth’s rotation around the sun (which produces seasons) and human cultural activities such as market days, royal coronations, tax collection, etc. Since the astronomical cycles aren’t neatly divisible and since the important holy days and so on shift from culture to culture and are changed by geographical migrations, crop cycles, wars, cultural diffusion and so forth, the outcome is a series of messy and somewhat inconsistent approximations that vary from place to place and between different cultures. There have likely been hundreds of different calendars over the course of history with slightly different ways of breaking up time into units such as weeks, months, and years, and modern timekeeping methods around the world smash together different aspects of different older systems.

While a number of variations have existed, there are three main calendar types: lunar calendars, such as the Islamic al Hijri calendar, with consist of 12 lunar cycles; solar calendars, such as the Gregorian calendar, which consist of a single solar cycle of seasons and are divided into semi-arbitrary months which don’t align with the phases of the moon; and lunisolar calendars, such as the Chinese and Jewish calendars, which have lunar months but a cycle of added months every few years so that holidays occur in roughly the same seasonal position year to year. There are other less common method as well, such as the Lakota year, which began with the first snowfall of the year, or the old Roman year, which had ten months of 29 or 31 days and then a set of “uncollected” days before the start of a new cycle, and therefore wasn’t consistent with either the solar or lunar cycle.

Once a society has adopted a method for measuring years (and deciding how much the length of a year will vary) then we reach the main portion of your question, which is what should be the starting point from which years are counted.

The Gregorian calendar uses the estimated year of the birth of Jesus as it’s starting point, but Christian year numbering took roughly 800 years to replace the late Roman method of counting from Diocletian’s reform of the empire, or from local regnal counting (ie, counting from the year that a political leader such as a king took office). Japan’s official calendar still uses the regnal year of the current emperor, making this year Reiwa 3 (the third year of Naruhito’s rule). As mentioned above, the Islamic al hijri calendar counts years from the Muslim community’s expulsion from Mecca, but Islamic kingdoms have also used regnal numbering at times. Often, multiple counts are in use at once within the same territory as different religious and civil calendars overlap. The Roman Empire for example had counts for the regnal years of an emperor, but also a longer count (Ab Urbe Condita) which counted from the presumed date of the founding of the city of Rome.

The Gregorian calendar is not the only calendar to pick an extrapolated date in the distant last as its starting point. Besides the Roman AUC calendar, there’s also the Jewish religious Anno Mundi calendar, which relies on medieval rabbinical calculations of the age of the world as the basis for its count (the current anno Mundi year is 5781, in case you’re curious). Some Theravada Buddhists also use a year system counting from the presumed date of Siddharta Gautama’s death.

What is exceptionally unusual about Gregorian reckoning is the practice, developed slowly between the sixteenth and nineteenth centuries if that calendar, of counting time backwards from before the presumed birth of Christ, rather than using separate chronological methods for discussions of events prior to year 1 of the calendar. This method of counting not only wasn’t used in ancient times, it wasn’t even used by Pope Gregory, who made the calendar reforms which established the Gregorian calendar. Using one reference point for all historical dates throughout time is convenient in one sense, because it makes it easy to discuss and compare the relative timeline of different historical developments in different regions, but it’s profoundly anachronistic and can give the false impression that people living in the time we call 200 AD or 400 BC would have used that counting method as well. Not only did they not use Gregorian dates, for most of history they wouldn’t have had the idea of a universally “correct” dating system and would have been far more likely to recognize multiple different calendar systems for different social purposes. Using reckoning from the (presumed) year of Jesus’ birth is an artifact of the power of Christian global hegemony during the colonial period, and has only stuck around because our global system is still politically and culturally Eurocentric, because an enormous amount of scholarship in the past 200 years had used this dating system, and because there’s no universally acceptable alternative calendar. If our continued use of Roman religious month names roughly 1700 years after the Roman Empire adopted Christianity is an indicator, however, we’ll quite possibly keep this dating system even if our society remains relatively secular compared to that of Pope Gregory.