BC dates in Python

Astronomers and aerospace engineers have to deal with BC dates and a continuous time line, so that's the google context for your search.

Astropy's Time class will work for you (and even more precisely and completely than you hoped). pip install astropy and you're on your way.

If you roll your own, you should review some of the formulas in Vallado's chapter on dates. There are lots of obscure fudge factors required to convert dates from Julian to Gregorian etc.

NASA Spice functions handle BC extremely well with conversions from multiple formats. In these examples begin_date and end_date contain the TDB seconds past the J2000 epoch corresponding to input dates:

import spiceypy as spice

# load a leap second kernel
spicey.furnsh("path/to/leap/second/kernel/naif0012.tls")

begin_date = spice.str2et('13201 B.C. 05-06 00:00')
end_date = spice.str2et('17191 A.D. 03-15 00:00')  

Documentation of str2et(), Input format documentation, as well as Leapsecond kernel files are available via the NASA Spice homepage.

converting from datetime or other time methods to spice is simple:

if indate.year < 0.0:
    spice_indate = str(indate.year) + ' B.C. ' + sindate[-17:]
    spice_indate = str(spice_indate)[1:]
else: 
    spice_indate = str(indate.year) + ' A.D. ' + sindate[-17:]

'2018 B.C. 03-31 19:33:38.44'

Other functions include: TIMOUT, TPARSE both converting to and from J2000 epoch seconds.

These functions are available in python through spiceypy, install e.g. via pip3 install spiceypy

Its an interesting question, it seems odd that such a class does not exist yet (re @joel Cornett comment) If you only work in years only it would simplify your class to handling integers rather than calendar dates - you could possibly use a dictionary with the text description (10 BC) against and integer value (-10) EDIT: I googled this:

http://code.activestate.com/lists/python-list/623672/