meeus | Implementation of `` Astronomical Algorithms '' by Jean Meeus | Computer Vision library

I did not look at the Fliegel-Van Flandern algorithm. I tested the remaining algorithm for the test cases Danny F requested, and added the last noon the Julian calendar was observed in Rome (JD 2299160) and the first noon of the Gregorian calendar in Rome (JD 2299161). For non-integer Julian days, I only used the Meeus algorithm since that's the only one that says it supports non-integers.

The internal algorithm was correct, when you allow for considering year 0 to not exist, and it always converts to Gregorian. The Meeus algorithm is correct, when you allow for it assuming the existence of year 0, and converting to Julian for October 4, 1582 and earlier, and to Gregorian for dates after that.

The Richards algorithm required corrections. Rather than using the floor function at certain points, I used the intdiv function in place of every "/" operator, which more closely follows Richard's description of the algorithm. After these corrections, the test cases were successful. (Richards considers the year 0 to exist and always converts to Gregorian.)

Meeus' Algorithm is actually in error (thank you Paul for pointing this out) there should be no 0 in Gregorian years, which means that all dates prior to Gregorian year 1 (i.e. Gyr < 0) are in error. Post Gyr (Gyr >= 1) are correctly converted. Also, he only has the convertor for dates post 1582G.

It seems therefore that the correct date is that listed by the jdtogregorian converter supplied by PHP.

As wOxxOm said, if you want to use ES6 modular system, you have to use Webpack or Rollup bundlers. Otherwise, you can refactor these modules to using globals and put them into manifest.json: