When using the Latin numbering system, which does not include zero, it's traditional to represent the years preceding 1 as "1 BC" etc. In this system the year 1 BC is a leap year.
When using a numbering system which includes zero, it's more convenient to include a year zero and represent earlier years as negative. This is the convention used in the "astronomical Gregorian calendar". In this system the year 0 is a leap year.
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