Ab Urbe Condita: The definition of the Empire’s Harmonious Calendar is this, counting from the founding – or, rather, counting from the winter solstice  that immediately preceded the founding (which was in the mid-to-late spring) in order that the start of the calendar year should be in a sensible place. And, technically, that year is numbered zero, not one, because discontinuities in the number line of years are annoying.
 If you were wondering how this ties into Eliéra’s non-standard what’s-axial-tilt-ness, consider it semantically equal to apodeuteros.
Alternative Calendar: Several. There’s the regular Harmonious Calendar, which is based on Eliera’s – as the homeworld and the throneworld – cycles, but also serves as the de jure calendar for the purposes of the financial year, official record-keeping, and suchlike. In turn, it’s based on weavetime, which is a universal time standard which simply expands the regular pulse (about 0.75s) into kilopulses, megapulses, gigapulses, etc., for scientific purposes, and also ease of making frame corrections. That standard has also spawned the difference between the absolute pulse (for values of absolute defined by a consensus of stargate timebase beacons), a.k.a. “empire time“, the local pulse, a.k.a. “wall-clock time“, and “journey time“, which is mission-elapsed time aboard lighthugger starships and thus dissonates quite remarkably from empire time. And, returning to more simple things, just about every planet, moon, drift, and city-ship has its own local calendar that matches up rather better with what its orbital cycles are actually doing.
“What time is it?” and “What day is it?” can be interesting questions to answer, sometimes.