> As https://en.wikipedia.org/wiki/Epoch_(reference_date)#Asian_national_eras

That's not really a single definition. ;-)

Also, as mentioned, the calculation seems to ignore leap years and such. Try to add test cases for `+2000-02-29T10:59:59Z` and `+2000-03-01T10:59:59Z`, they both return `64055444399`.