
What Time Is It? - edent
https://shkspr.mobi/blog/2016/08/what-time-is-it/
======
masklinn
> There are 11 missing days in 1752.

There aren't 11 missing days in 1752 though, there are 11 missing days in 1752
_in the british calendar_. As the original source notes, different countries
switched to the gregorian calendar at different times, thus the "missed days"
vary wildly even if you ignore Sweden (tried to switch gradually by skipping
leap days, forgot to skip some, then switched back to julian, then switched to
gregorian at once) and many Orthodox countries remained on Julian well into
the 20th century (hence the October Revolution starting early November 1918,
it was October julian) and had to skip 13 days.

~~~
mikeash
Sweden is a great case. To resync after their failed attempt to switch, they
included a February 30th in one year, so that is actually a valid date, in one
year, in one place.

Alaska is another fun case. They switched calendars and simultaneously
switched which side of the date line they were on, with the effect that Friday
October 6th was followed by Friday October 18th.

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gpvos
One tidbit I didn't know yet, but was linked from this article: Britain wasn't
actually on GMT at 1 January 1970:
[http://markfrimston.co.uk/articles/34/epoch-
zero](http://markfrimston.co.uk/articles/34/epoch-zero)

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dpcx
To continue the thought... What about time on different planets? Our time is
based on solar revolutions (mostly). If Mars had the same, their time would be
different from ours very quickly.

~~~
kazagistar
Computers don't care much, since they will just keep using seconds since 1970,
which is the same everywhere.

However, if you start regularly traveling quickly though space, the question
of aligning offsets caused by relativity could get problematic.

~~~
gpvos
_> seconds since 1970_

In UTC or TAI? (I.e., with or without leap seconds?)

~~~
masklinn
UTC.

You can use TAI as the base for your system, but it'll be non-conformant (and
you'll drift out of sync with the rest of the world obviously).

~~~
gpvos
...but if you use UTC, as is standard, it's not exactly "seconds since 1970",
as kazagistar claimed (and as you explain in a nearby comment), which was my
point.

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niccaluim
I was about to mention that the DDHHMMZ format is used to stamp aviation
weather reports and forecasts too, but I see the article beat me to it. :)
Anyway here's the current (as of this comment) report from SFO if you're
curious about what they look like:

    
    
      KSFO 301656Z 29011KT 10SM FEW008 SCT160 19/13 A3002 RMK AO2 SLP165 T01890128
    

You can see the timestamp in the second element. Once you learn the format
it's second nature, really.

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wallacoloo
I'm no astronomer, but as to the article's closing question, I imagine there's
_some_ unambiguous solution based on the relative locations of celestial
bodies. For example, supply the angle between earth -> sun -> Mars, and
however other many data points are needed. With enough angles, I think this
would _also_ establish location, due to the propagation velocity of
information.

~~~
crispyambulance
Calendar time, when it is done truly right, is a complicated topic. There's a
nice book that has way more detail on this than most people actually would
want (Calendrical Calculations, [https://www.amazon.com/Calendrical-
Calculations-Nachum-Dersh...](https://www.amazon.com/Calendrical-Calculations-
Nachum-Dershowitz/dp/0521702380)).

~~~
edent
Ooh, that book looks great. Thanks, will add it to the post.

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losteverything
It hasn't even been 100 years since US congress had the standard time act of
1918. Visiting Golden Spike NP taught me the railroads created standard time
from a good exhibit at the park's museum. In 1883 if I recall. Until then time
in town A did not need to be the same as time in town AA.

But my favorite answer still is, "Time to get a watch."

------
brohee
The article could have been better citing RFC 2550 "Y10K and Beyond" which
despite being a joke (April 1st) RFC provides a completely universal date
format (can describe any date present and future, with as much precision as
you want), that are orderable by their lexical sort order.

Yet present day time representation are easily parsed by regular humans.

