[sdnog] Impact of new timezone in Sudan

[Philip Paeps]

On 2017-11-01 00:31:30 (+0300), sara alamin wrote:
>Some one asked me this:
>
>How can I push new time zone to all computer ( clients) from Active
>Directory (domain) ?

If Microsoft releases a patch with updated timezone information, you can 
just push that like any other (security) update?  Presumably you'll have 
to jump through the usual hoops to get your hands on the update if 
Microsoft doesn't like Sudan.  In unmanaged environments, Windows Update 
should fetch the update from Microsoft (again, possibly via a proxy).

Note that you're not actually moving to a new timezone.  Your timezone 
is still Africa/Khartoum.  It's just that $someone noticed that the sun 
was slightly off-centre at noon and it was easier to change your 
timezone's offset from UTC than to move the sun (or the earth)[*].

In other words: you don't want to change people's timezone.  You want to 
update people's definition of the timezone they know and love.

What you *really* shouldn't do (certainly not to unsuspecting users!) is 
tell their computers to pretend they're somewhere else so the local time 
is correct.

If you tell people's computers to pretend they're in (for instance) 
Africa/Maputo instead of Africa/Khartoum, the current time may look 
correct, but historical timestamps and comparisons between timestamps 
will be wrong any time Africa/Maputo was different from Africa/Khartoum.  
This will lead to all sorts of exciting surprises.

For example...

Time in Khartoum went:

Tue Oct 31 23:59:58 2017 EAT
Tue Oct 31 23:59:59 2017 EAT
Tue Oct 31 23:00:00 2017 CAT
Tue Oct 31 23:00:01 2017 CAT

In Maputo it went:

Tue Oct 31 22:59:58 2017 CAT
Tue Oct 31 22:59:59 2017 CAT
Tue Oct 31 23:00:00 2017 CAT
Tue Oct 31 23:00:01 2017 CAT

If you try to compare timestamps, you'll find an hour missing.  You 
might be able to remember (at least for the next few days or weeks) that 
31 October 2017 was an hour longer than other days, but will you 
remember that all of October 2017 was an hour longer or that all of 2017 
was an hour longer?

There have been other historical changes in Africa/Khartoum too.  Take a 
look at the output of `zdump -v Africa/Khartoum` and compare it to the 
output of `zdump -v Africa/Maputo`.

It goes without saying that you don't want to change the definition of 
UTC either.  That leads to even more excitement.

Computers are a lot better at remembering these things but they only way 
for them to remember is if you keep them in the correct timezone and 
make sure that the definition of that timezone matches reality.

Isn't time fun? :)

All this to say: Please don't change people's clocks or people's 
timezones.  Patch their timezone definitions!

Philip

[*] Every time someone tinkers with timezones, I wonder if it wouldn't 
be simpler to move the earth instead -- or the sun!

-- 
Philip Paeps
Senior Reality Engineer
Ministry of Information