On Thu, 2013-09-26 at 10:54 -0700, Daniel Hedlund wrote: > On Wed, Sep 25, 2013 at 10:54 PM, Dale Snell <[email protected]> wrote: > > > Those March days are all Daylight Saving Time switchover days, aren't > > they? (If they're not, I'm going to look really stupid...) There is > > no 2:00AM hour on those days, thus the dates really are invalid. > > > > Dale is correct here in that they really aren't valid dates due to the > daylight savings change. To compound the problem, dates between 01:00:00 > and 01:59:59 are ambiguous if you don't specify a timezone.
I have a side project that deals with historic time data over multiple timezones, and its tricky to say the least, even with libraries to do the heavy lifting. Its kind of a notoriously tricky area where naive solutions can go very wrong. It's an interesting subject. Further reading below if anyone is interested, I would particularly recommend A literary appreciation of the Olson/Zoneinfo/tz database. Paul M Further Reading: =========================================================== (from the python timezone library documentation) In cases where countries change their timezone definitions, cases like the end-of-daylight-savings-time occur with no way of resolving the ambiguity. For example, in 1915 Warsaw switched from Warsaw time to Central European time. So at the stroke of midnight on August 5th 1915 the clocks were wound back 24 minutes creating an ambiguous time period that cannot be specified without referring to the timezone abbreviation or the actual UTC offset. In this case midnight happened twice, neither time during a daylight savings time period http://pytz.sourceforge.net/ (from the gnu manual on date and time) First, a quote: Our units of temporal measurement, from seconds on up to months, are so complicated, asymmetrical and disjunctive so as to make coherent mental reckoning in time all but impossible. Indeed, had some tyrannical god contrived to enslave our minds to time, to make it all but impossible for us to escape subjection to sodden routines and unpleasant surprises, he could hardly have done better than handing down our present system. It is like a set of trapezoidal building blocks, with no vertical or horizontal surfaces, like a language in which the simplest thought demands ornate constructions, useless particles and lengthy circumlocutions. Unlike the more successful patterns of language and science, which enable us to face experience boldly or at least level-headedly, our system of temporal calculation silently and persistently encourages our terror of time. ... It is as though architects had to measure length in feet, width in meters and height in ells; as though basic instruction manuals demanded a knowledge of five different languages. It is no wonder then that we often look into our own immediate past or future, last Tuesday or a week from Sunday, with feelings of helpless confusion. ... —Robert Grudin, Time and the Art of Living. https://www.gnu.org/software/coreutils/manual/html_node/Date-input-formats.html#Date-input-formats http://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/ http://www.quirksmode.org/blog/archives/2009/04/making_time_saf.html http://naggum.no/lugm-time.html http://stackoverflow.com/questions/6841333/why-is-subtracting-these-two-times-in-1927-giving-a-strange-result/6841479#6841479 -- Technical Support Specialist, Free Geek Free Geek Tech Support: [email protected] (503) 232-9350 option 6 Tuesday-Saturday: 12-1,1:30-5:45PM _______________________________________________ PLUG mailing list [email protected] http://lists.pdxlinux.org/mailman/listinfo/plug
