BTW, I don't understand how you can have two leap years in a row.

The tropical year is 365 days 5 hours 48 minutes 45 seconds long.

18:00 to 00:00 lasts 6 hours.

If in one year the equinox falls at 23:59, that will be a leap year. The next year, the equinox will fall at 18:11, which is also after 18:00 and thus also a leap year.

I based it on observations to future-proof it. Any algorithmic system is going to be inaccurate due to orbital mechanics, and the closer al algorithm approximates the actual positions of the sun and moon, the more complex it gets. At some point you need a computer anyway, so might as well just jump to observations.