old style.The system of ordering time according to the Julian method, introduced by Julius
Caesar in 46 B.C., by which all years have 365 days except the years divisible by 4, which have 366 days. • This differs from the modern calendar in that it assumes that there are exactly 365.25 days in a year. But there are actually slightly less than 365.25 days in a year, so the old-style calendar adds too many days over time. The Julian calendar was reformed by Pope Gregory XIII in 1582. — Abbr. o.s. — Also termed Julian calendar. Cf. NEW STYLE.
[Blacks Law 8th]