Astronomical Gregorian Calendar to Jewish Calendar Conversion

Please permit zero and negative Gregorian years so the calculator can function as an "astronomical" Gregorian-to-Jewish calendar converter.  Astronomy requires contiguous years and although 1 BC jumps to 1 AD, for the purpose of astronomy, 1 BC is year 0, 2 BC is year -1, etc.

By the way, thank you for not including the 10-day Gregorian transition since your converter works as an "astronomical" Gregorian calendar converter for Gregorian dates before the transition (not interested in Julian dates in history, just Gregorian dates for astronomy).  We just need the converter to accept year 0 and negative years.  Thanks!