[Tutor] Time - sum/difference

[Alan Gauld]

> Anyway, I found a library for working with time zones
> http://pytz.sourceforge.net/

Thanks for pointing this one out, it looks good.

As someone who once commisioned a research paper on Time Zones and
programming I must say this looks like one of the most complete timezone
sites around. However this statement needs to be highlighted with an
anecdote :-)

"Note that if you perform date arithmetic on local times that cross DST 
boundaries,
the results may be in an incorrect timezone "

DST is not an agreed well defined entity. When on a skiing holiday in the 
tiny
country of Andorra the local council decided that the snow was melting
too much in the late afternoon.

Their brilliantly simple solution was to bring forward DST by a week
(a simple announcement on local radio and newspaper!) and the snow
slopes now closed an hour earlier saving the snow from skiers turning
it to slush in the late afternoon sun!

I can't imagine the havoc that created for their computer systems...
But tourism is king in Andorra.

A salutory lesson on how fragile is our grasp on time.

Alan G.