The calendar system of Polysiton is a simple one, and it is used primarily by the Lagikoi and the Sentinels to keep record in accordance with the Polysitonian orbiting period.
The humans use the Gregorian System in tandem with the Polysitonian calendar, primarily because the Gregorian System is the common timekeeping system in the human world and probably the only one that has some basis to be a common timekeeping system. Nonetheless, this would be rather inconvenient when it comes to local events, so they have created a simplified calendar from what the Lagikoi uses.
Old Lagikoi CalendarEdit
The Lagikoi divided their planet's orbital year into 390 days in accordance with the orbital period. It is an unusually convenient system, since Lampardion's orbital period is almost exactly equal to 390 Polysitonian days. Unlike the Meizotere Hellas's calendar that subdivided its days into 13 months of 39/40/41 days each, the old Lagikoi system divided its days into 3 subperiods of 130 days each, further subdividing that into 13 deciles of 10 days each. The subperiods were named Proteros, Deuteros, and Hysteros respectively, so the 241st day of a year would be the 1st day of the 11th Decile of Deuteros.
While the Lagikoi system was easy to use, the problem came with attempting to convert the date into Gregorian calendar system. To further faciliate the conversion process, the Sentinels simply flat-out just use the day of the year without any subdivision. Needless to say, this calendar system isn't exactly popular with the Lagikoi, many preferring the old system.
As for the naming of the year, this is one aspect of the system that the Lagikoi particularly liked. Before the First Polysitonian War, they used A.T.C., or At the Time of Colonization as the benchmark. However, this would change after the said war, to which the new and more popular B.B.O./Y.B.O./A.B.O. is used, meaning Before/Year of/After the Battle of Oxys respectively.