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Why do most watches with the perpetual calendar not cover years after 2099?


The year 2000 was a special leap year. A year whose last two digits are divisible by 4 is a leap year and its days become 366 by an extra day being added.
However, there is a supplemental definition for the leap year as follows:
・A year whose last two digits are 00 is NOT a leap year except for the year is divisible by 400.

The year 2000 has 00 as the last two digits but it is divisible by 400, making it a leap year. Consequently, the years from 1901 to 2099 is a little rare 199-year period when a leap year comes every four years with no exception. (The next period of the same kind is from 2301 to 2499, about 300 years later.)
As a result, it is regarded sufficient for most of computers and other devices to be equipped with the calendar with the simple leap year rule that "A year divisible with 4 is a leap year." and that causes no problems in most cases.
However, the year 2100 is not a leap year as it is divisible by 4 but not by 400 and ends with 00 and the simple rule above will become ineffective and the calendar will become incorrect.