Calendar math problems

[Jaap Spies]

Ben Logan wrote:

> Hi,
> 
> I need to be able to add/subtract an arbitrary number of days to/from
> an arbitrary date.  The days and dates are all within the range
> 1970-2038, so I thought I would convert everything to Unix time
> stamps, do the addition/subtraction on integers, and convert the
> result back to a date tuple.
> 
> Is this the best way to accomplish the task?  How would one go about
> it if the dates fall outside the timestamp range?  I looked at the
> calendar module functions, and they would be helpful for writing an
> algorithm to do the dirty work, but I don't see any really easy
> solution.
> 
> Thanks,
> Ben
> 
> 

You should use the astronomers trick: translate date/time to Julian Day 
(JD): a continuous count of days and fractions thereof since the year
-4712. Do what you want to do and then translate back.

There are various sources on the internet.
Python source: http://astrolabe.sourceforge.net/ see calendar.py:
cal_to_jd() and other usefull algorithms.

Jaap Spies
http://pynovas.sourceforge.net/