noun
Julian calendar
Refers to the calendar introduced by Julius Caesar, used before the Gregorian reform. Often contrasted with 新太陽暦 (Gregorian calendar).
See also: ユリウス暦
ロシア革命は旧太陽暦の十月に起きた。
The Russian Revolution occurred in October of the Julian calendar.
Compound of 旧 (old) + 太陽暦 (solar calendar). The term directly describes the older solar calendar system.