Someone told me that https://en.wikipedia.org/wiki/Tz_database (tzdata, zoneinfo, IANA time zone, timezone, olson database) contains the necessary information about different Julian calendars so one can convert them to Gregorian. Yes this seems unlikely given that this would not fit into the normal structures for information on one time zone.

AFAIK https://en.wikipedia.org/wiki/ICalendar can handle Julian dates.

Investigate if either is true for historic dates and if we can avoid writing our own conversion code. Perhaps there is a useful implementation that can be reused.

The PHP conversion function cal_to_jd( CAL_JULIAN, $m, $d, $y ); only supports a Julian calendar starting on January 1st, none of the others.