[Datetime-SIG] DST explained visually

Tim Peters tim.peters at gmail.com
Wed Aug 26 22:28:11 CEST 2015

> Which reminds me, there are some edge cases to consider. What's the
> local time for UTC=A? And for UTC=C? I guess the rule is to use half-open
> intervals on the X axis that are open on the right, so that A maps to Q and
> C maps to R.

This is clearer ;-) using the hyperreal number line:


Transitions theoretically happen at A-h and C-h, where h is any
infinitesimal > 0 (any hyperreal number strictly greater than 0 and
strictly less than any real number).  So your intuition is right.

You may ask "but what are the local times corresponding to A-h and
C-h?".  That would just be making trouble for no good reason.  There
are far too few points on a real number line to display A-h or C-h, so
"who cares?" is appropriate for _many_ reasons ;-)

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