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
The Russian Revolution occurred in October of the Julian calendar.
Synonym; ユリウス暦 is a direct loan from 'Julian calendar' and is equally common in academic contexts.
Refers to the Gregorian calendar, the modern solar calendar that replaced the Julian calendar.
Compound of 旧 (old) + 太陽暦 (solar calendar). The term directly describes the older solar calendar system.