How the Tz Database Works (2020)

Posted4 months agoActive4 months ago

jumbosushi

82 points

16 comments

yatsushi.comTechstory

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Time Zone ManagementTz DatabaseDate and Time Handling

Key topics

Time Zone Management

Tz Database

Date and Time Handling

The post explains how the tz database works, sparking a discussion about its history, usage, and interesting aspects, including personal anecdotes and related resources.

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01Story posted
Sep 7, 2025 at 5:58 PM EDT
4 months ago
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02First comment
Sep 11, 2025 at 7:39 AM EDT
4d after posting
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14 comments in 84-96h
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Sep 12, 2025 at 7:34 AM EDT
4 months ago
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Discussion (16 comments)

Showing 16 comments

wodenokoto

4 months ago

2 replies

Is the author using `/ #` as the prompt string? I find the commandline sections hard to read.

johannes1234321

4 months ago

If you look at the first command - there he switches into a docker container, where archlinux seems to use that as root prompt.

arccy

4 months ago

sounds reasonable: `#` is the standard prompt marker when you're root, `/` is the path they're currently at.

ape4

4 months ago

2 replies

Since nobody else has posted it...

https://xkcd.com/2347/

dahauns

4 months ago

It's not really applicable here, though - being an official IETF/ICANN/IANA joint.

I mean, it has its own RFC called "Procedures for Maintaining the Time Zone Database": https://www.rfc-editor.org/rfc/rfc6557

I'd say that's as far from "some random person in Nebraska" as you can get...

(And if you'd do a representative survey among developers having had to deal with time zone shenanigans, I'm fairly certain "thanklessly" won't hold either :) )

adolph

4 months ago

explainxkcd is timing out, probably from all of HN checking their recollections that 2347 was about the tzdb.

https://www.explainxkcd.com/wiki/index.php/2347:_Dependency

jlev

4 months ago

1 reply

This change https://github.com/eggert/tz/commit/baea52df7ac9c4b53857556f... was a huge pain in my ass in 2013.

I was working in Libya on voter registration tools with the UN and the High National Election Commission. The government decided to not implement a planned TZ change, and didn't inform the public until the day of. Not the hardest thing we dealt with, that was a full country internet shutoff by a mob outside our data centre (https://www.bbc.com/news/world-africa-25481794). Sometimes the politics of a project are more complicated than the technology...

We did implement an all-SMS voter registration system, which was pretty cool. Hasn't been used much since, but it's all open source. https://github.com/hnec-vr

ta1243

4 months ago

2 replies

> I was working in Libya

Complete tangent, but I don't think many Americans know this (I'm assuming you're an American)

If you aren't American, you now are now ineligible to go to America as a tourist without an expensive hasslesome visit to a US embassy. (No online ESTA)

I have friends that have gone to countries like Libya, and Syria to do similar international work. An British engineer I know recently went to Syria for a few days.

I pointed out that he is no longer allowed to go to the US without going to the embassy for a visa. He's ineligible for an ESTA.

He said "fine, work will have to pay for it".

I then pointed out this is for the rest of his life. He regularly holidays in Florida. He might leave the media or change jobs so they no longer pay for a visa.

I've been asked to go to Iraq in the past, but I've said no because of this. Was a very expensive weekend for my friend.

Another friend is in the British Army, he's gone to various places as part of both British and NATO deployment, not using his personal passport - but using travel orders. He managed to avoid going to Iraq which is lucky for him, means he can still get an ESTA.

jlev

4 months ago

1 reply

I am an American, and most of the other folks on the project were as well. I did go through an interview with Customs officers when I returned to the US, but it didn't affect my ability to use GlobalEntry. I don't have clearance, but it might cause questions if I did apply for that in the future.

I've also been to Syria as a tourist several times, and at one point had to maintain a second passport for visiting Israel or the West Bank. You can't travel to most of the Arab world if you have Israeli stamps, but you can get another book from the US to keep them separate.

Sorry for your friend. Borders are bullshit.

ta1243

4 months ago

Second passport is fine, but they stopped stamping at Tel Aviv many years ago. I still got a stamp last time I went to gaza, but that was about a decade ago and pre current passport. Obviously not a concern now)

elAhmo

4 months ago

Many Americans (and non-Americans) also don't know that not all countries are supported with ESTA. So, for me, being a citizen of a '3rd world country' in Europe, I have to visit the embassy.

Although I am resident in EU and haven't been to any of those 'flagged' countries.

foresto

4 months ago

Obligatory Tom Scott video:

https://www.youtube.com/watch?v=-5wpm-gesOY

knagy

4 months ago

One interesting thing I learned while I tried to handle local time and NTP on a Raspberry Pico is that the TZ environment variable (that's usually something like "Etc/UTC" pointing to a zoneinfo file) can contain a TZ config as well, like "CET-1CEST,M3.5.0/2,M10.5.0/3". [1]

[1] https://sourceware.org/glibc/manual/2.42/html_node/TZ-Variab...

shagie

4 months ago

I've found the comments in the database files (e.g. North America https://github.com/eggert/tz/blob/main/northamerica ) to be an interesting documentation of the history of the definition of time for various areas. For example, Indiana https://github.com/eggert/tz/blob/main/northamerica#L846

chaidhat

4 months ago

Took a class with professor Paul Eggert at UCLA, the person who maintains this db. Super chill dude. In his exams, he makes sure to add a question he doesn't know the answer to himself.

jjice

4 months ago

Great write up. This is very informative. Yet another thing I've taken for granted and hope to continue to take for granted because it generally just works and the bugs are my own.

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