Maybe this site will help:
 
http://www.calendarhome.com/converter/
 
I like the convention when using Gregorian extrapolated backward (before 1582 AD) to have a 0 year.  While when using Julian, one does not count from nothing as we do today.  And technically one doesn't go negative with Julian, just 2 AD, 1 AD, 1 BC, 2 BC, etc.
 
This convention means the significant eschatological date 445 BC Julian is -444 Gregorian, a significant triple number to go with a significant date.