On Wed, Dec 06, 2023 at 11:46:50PM -0600, David Wright wrote:
> On Wed 06 Dec 2023 at 18:16:42 (-0500), Pocket wrote:
> > Which BTW this whole discussion about timezones is just water over the dam.
> >
> > The system should be set to UTC, the "timezone" issue is really just a
> > "human" issue as the UTC clock is always correct
>
> While I'm glad we're not discussing whether or not the RTC is set to
> UTC or TAI or local time, I do want my computers to display to /me/ the
> correct date and time corresponding to my location. And when I travel,
> I expect my phone to switch its display automatically, using some
> reasonably up-to-date tables.
I haven't had time to read and follow up on Pocket's list of references,
but I'd like to respond to these points with a real anecdote.
One of our systems at work uses a database with a web front end, where
users input the starting and ending times of medical tests that have
been performed on a patient. When the tests are finished, and all the
data have been entered, billing charges are generated, and these charges
depend on the length of the test.
You'd think that you can determine the length of the test by subtracting
the start time from the end time, right? Unfortunately, that fails
two times a year, if you don't do it exactly right. The naive approach
of simply looking at the time field ("test started at 01:45 and ended
at 03:15 the same day, so it must have lasted 90 minutes") is wrong.
You have to look at the entire date-plus-time as a single timestamp,
and interpret it within the correct time zone. That test might have been
90 minutes, or it might have been 30 minutes, or it might have been 150
minutes, depending on whether a DST transition happened in that interval,
and which way the clock moved.
I *literally* had to fix that bug (in March). This isn't hypothetical.
Of course, the system I'm dealing with only covers tests that have been
performed recently, not tests from a century ago. So the historical
interpretation of times under previous government DST rules *is*
hypothetical as far as this anecdote goes. But I hope some of you can
appreciate it nonetheless.
