Soleian years begin on the Spring Equinox, and are divided into the four traditional seasons of spring, summer, and fall. Seasons are further split up into three parts, which makes up the months. Years are 364 days long. Unlike in our world, this perfectly aligns with the sun’s yearly cycle and makes leap years unnecessary.
The year system, referred to as S.A., began when Soleia officially (more or less) became its own nation. The bulk of String of Stardust takes place in the late spring and summer of 3062.