The proper name for the calendar using the Gregorian system before the start of the Gregorian calendar is "the proleptic Gregorian calendar".
And indeed that's Python's calendar:
'A date object represents a date (year, month and day) in an idealized calendar, the current Gregorian calendar indefinitely extended in both directions.'
'This matches the definition of the “proleptic Gregorian” calendar in Dershowitz and Reingold’s book Calendrical Calculations.'
https://docs.python.org/3/library/datetime.html#id2 https://docs.python.org/3/library/datetime.html#id5
Whereas Ruby has proleptic Gregorian Time, but civic DateTime:
https://ruby-doc.org/stdlib-2.6.1/libdoc/date/rdoc/DateTime....