Ask what year it is and you will get one answer almost everywhere. In a handful of countries, the honest answer is "which calendar are you asking about?"
Bangkok — Thailand is the only country in the world using the Buddhist Era as its official calendar. The epoch is the parinibbāna of the Buddha, traditionally placed at 543 BCE, so Thai years run 543 ahead. The Gregorian year 2026 is 2569 BE. Everything underneath is Gregorian — same months, same leap years — only the year number differs. And until 1941, the Thai year began on 1 April.
Taipei — Taiwan numbers years from 1912, the founding of the Republic of China, so 2026 is Minguo 115. Subtract 1911 from the Gregorian year. The idea follows the old Chinese imperial habit of counting by the reigning emperor's era; with no emperor, the founding of the republic became the era instead.
Tokyo — Japan runs the same imperial logic, but keeps changing eras. Each new emperor starts a new one and the count resets to one. The current era, Reiwa, is the 232nd, and began on 1 May 2019. So 2026 is Reiwa 8. Because an era ends when a reign ends, and reigns do not wait for January, the Western year 2019 was two Japanese years: Heisei 31 until 30 April, Reiwa 1 from 1 May.
Tehran — Iran uses the Solar Hijri calendar, which is arguably the most accurate civil calendar in use anywhere. It has no leap-year formula at all. Each year begins at the exact instant of the vernal equinox, astronomically determined for the meridian at 52.5° east; whether the final month gets 29 days or 30 is decided by how many days actually elapsed between one equinox and the next. The best mathematical approximation of it drifts by one day in about 110,000 years. The Gregorian calendar drifts by one day in about 3,236.
Riyadh — Saudi Arabia uses the lunar Hijri calendar, twelve months of 29 or 30 days set by the sighting of the moon, coming to about 354 days. Because that is eleven days short of the solar year, Islamic dates walk steadily backwards through the seasons and return to where they started after roughly 33 years. Anyone who fasts through a lifetime will experience Ramadan in every season there is, and see the full cycle come round only once.
There is a lesson buried in this collection. Three of these five countries — Thailand, Taiwan, Japan — keep a Gregorian structure and change only the number. Two — Iran and Saudi Arabia — change the structure itself. And Saudi Arabia demonstrated in 2016 exactly how much a calendar can be worth: by moving public sector salaries from the Hijri year to the Gregorian one, the government gave every civil servant an eleven-day pay cut without altering a single figure on a payslip.