[Datetime-SIG] DST explained visually

Tim Peters tim.peters at gmail.com
Wed Aug 26 22:06:21 CEST 2015


[Guido]
> I've drawn a simple diagram showing the relationship between UTC and local
> time throughout a DST cycle:
>
> https://www.dropbox.com/s/ptx58d9zkd7m4vj/2015-08-26%2010.28.38.jpg?dl=0

Cool!  But in the spirit of mailing lists, I want to complain about
the asymmetry of the labels:  A, B, C, D are the first four letters of
the English alphabet, so for symmetry it's just plain broken that you
didn't use W, X, Y, Z for the other labels ;-)

Another thing to note:  as Isaac observed, while UTC->local is a
function, that's not as exploitable as one might hope, because it's
not a continuous function.  However, the diagram as a whole shows a
collection of 3 piecewise continuous bijections (each solid diagonal
line segment is a one-to-one continuous function "in both directions"
- and also monotonic).  That's highly exploitable.

Indeed, for times through 2037, tzfiles explicitly store all points
akin to A and C (the UTC points bounding the piecewise continuous
bijections) in a sorted list.  It's a minor annoyance that there are
an infinite number of such points ;-)


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