Answer:
The earlier Julian calendar was in use prior to the adoption of the current Gregorian calendar. It turned out to be remarkably close to the actual length of the year, but the Julian calendar wasn't so accurate that it didn't gradually veer off course over the centuries. But hundreds of years later, only monks had any free time for intellectual activities, and they were forbidden from considering the concept of "secular time" for any reason other than determining the date of Easter. The study of time measurement was formerly regarded to be overly intrusive into God's personal business throughout the Middle Ages and was later dismissed as a menial, mechanical subject undeserving of serious attention.
Therefore, it wasn't Pope Gregory XIII (1502 – 1585) eventually changed the Julian calendar in 1582, by which time Caesar's calendar had strayed a full 10 days off track. Ironically, by the time the Catholic church gave in to the pressure of the scientific evidence that exposed the error, it had largely lost the ability to put the repair into effect. Gregory's calendar prompted Protestant tract writers to label him the "Roman Antichrist" and assert that its underlying objective was to prevent sincere Christians from worshiping on the appropriate days. It took until well into the 18th century for Europe as a whole to formally adopt the "new" calendar as we know it today.