Refers to the act or process of changing or adjusting a calendar system, such as the adoption of the Gregorian calendar.
In Japan, a calendar reform was carried out during the Meiji period.
改暦 is a more common and concise term for calendar reform, while 歴法の改正 is a longer, more formal compound.
Compound of 歴法 (calendar system) and 改正 (reform, revision).