Pope Gregory XIII. adopted, the older Julian calendarwas used

Before today’s Gregorian calendar was adopted, the older Julian calendarwas used. It was admirably close to the actual length of the year, as it turns out, but the Julian calendar was not so perfect that it didn’t slowly shift off track over the following centuries. But, hundreds of years later, monks were the only ones with any free time for scholarly pursuits – and they were discouraged from thinking about the matter of "secular time" for any reason beyond figuring out when to observe Easter. In the Middle Ages, the study of the measure of time was first viewed as prying too deeply into God’s own affairs – and later thought of as a lowly, mechanical study, unworthy of serious contemplation.

As a result, it wasn’t until 1582, by which time Caesar’s calendar had drifted a full 10 days off course, that Pope Gregory XIII (1502 - 1585) finally reformed the Julian calendar. Ironically, by the time the Catholic church buckled under the weight of the scientific reasoning that pointed out the error, it had lost much of its power to implement the fix. Protestant tract writers responded to Gregory’s calendar by calling him the "Roman Antichrist" and claiming that its real purpose was to keep true Christians from worshiping on the correct days. The "new" calendar, as we know it today, was not adopted uniformly across Europe until well into the 18th century.