While Aloisius’ calculations are impressive, he was standing on the shoulders of giants.

Julius Caesar introduced the Julian Calendar in 45 BC, and it was used in Europe & elsewhere for the next 1600 years (or nearly 2,000 years in total if you take into account countries such as Russia that refused to switch until the early 20th Century). This calendar used calculations by a number of mathematicians and astronomers, including Sosigenes of Alexandria and (for the Romans at least) it was the first calendar that required no adjustment.

Previous to this, the Romans had had a calendar that was shorter than the year by several days, and so every couple of years the Pontifex Maximus would add an extra ‘intercalary’ month to make up. Yes, it was as inexact as it sounds, and the year was often shortened or lengthened for political benefit. Caesar recognised this, took full advantage of it, and then left a legacy of a calendar that didn’t need this sort of hasty correction. If you’re interested in that bit you should read about the year 46BC.

But over time it became obvious that Caesar’s calendar wasn’t perfect. The year was too long and the Equinoxes & Solstices, which are supposed to occur at roughly the same dates each year, were gradually moving.

Therefore Aloisious’ calculations, while impressive, had the benefit of the fact that he didn’t need to calculate the year from scratch - he only needed to work out how far wrong they had gone, and how long it had taken, in order to correct the calculation on which Caesar’s calendar had relied.