310s BC
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1 BC 0001, year

The year zero does not exist in the Anno Domini (AD) system commonly used to number years in the Gregorian calendar and in its predecessor, the Julian calendar . In this system, the year 1 BC is followed by AD 1 . However, there is a year zero in astronomical year numbering (where it coincides with the Julian year 1 BC ) and in ISO 8601:2004 (where it coincides with the Gregorian year 1 BC), as well as in all Buddhist and Hindu calendars.

The Anno Domini era was introduced in 525 by Scythian monk Dionysius Exiguus (c. 470–c. 544), who used it to identify the years on his Easter table. He introduced the new era to avoid using the Diocletian era, based on the accession of Roman Emperor Diocletian, as he did not wish to continue the memory of a persecutor of Christians. In the preface to his Easter table, Dionysius stated that the "present year" was "the consulship of Probus Junior " which was also 525 years "since the incarnation of our Lord Jesus Christ ". How he arrived at that number is unknown.

The Old Terran Time format is based on the Pre Astro BC/AD system and also has no Year Zero

In the timekeeping format UST (Union Standard Time ) Year 0 or 0000 UST is the old Year 2220 OTT and marks the beginning of the Union .
While UST has been accepted and officially in use since 5100 Replacing both OTT and the short lived, never adapted NUT (New Union Time ) and citizens now use Standard time for dates. The years remain being expressed in OTT .

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