Beyond The Impossible runs as close to real time as possible. While in-story a single issue might last for a single day and storylines are published across several months, different time skips between issues (or even in the middle of them) eventually pick up the difference.
This means that, on average, an issue typically takes place in the year it is published.

Within the story, while Earth still follows the Gregorian Calendar, the rest of the Galaxy follows different calendars.

Age of Null

Followed by the Church of Null and eventually becoming the standard of the Olympian Galaxy.
Year 1 celebrates the birth of Noriko Null.
Note that there is no “year zero”: while Null receives her powers when she is 18 years old, it happens in 19 AD.
1 AD (After Null) is 1996 AD
1 BD (Before Null) is 1995 AD

Age of Olympus

Seen in Untold Tales of Vesta.
Year 1 celebrates the establishment of the feudal system of the Olympian Galaxy.
1 AO (Age of Olympus) is 12,056 BC