Re: [Datetime-SIG] DST explained visually
[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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Tim Peters