How to Think About Time in Programming

Time is a mess. Always. The author only scratched the surface on all the issues. Even if we exclude the time dilation of relativity which affects GPS/GNSS satellites - independent of if it is due to difference in gravitational pull or their relative speed over ground, it's still a mess.

Timezones; sure. But what about before timezones got into use? Or even halfway through - which timezone, considering Königsberg used CET when it was part of Germany, but switched to EET after it became Russian. There's even countries that have timezones differenting by 15 minutes.

And dont get me started on daylight savings time. There's been at least one instance where DST was - and was not - in use in Lebanon - at the same time! Good luck booking an appointment...

Not to mention the transition from Julian calendar to Gregorian, which took place over many, many years - different by different countries - as defined by the country borders at that time...

We've even had countries that forgot to insert a leap day in certain years, causing March 1 to occur on different days altogether for a couple of years.

Time is a mess. Is, and aways have been, and always will be.

Author covers how IANA handles Königsberg, it is logically its own timezone.

  An IANA timezone uniquely refers to the set of regions that not only share the same current rules and projected future rules for civil time, but also share the same history of civil time since 1970-01-01 00:00+0. In other words, this definition is more restrictive about which regions can be grouped under a single IANA timezone, because if a given region changed its civil time rules at any point since 1970 in a a way that deviates from the history of civil time for other regions, then that region can't be grouped with the others

I agree that time is a mess. And the 15 minute offsets are insane and I can't fathom why anyone is using them.

Yep. Fortunately, a lot of apps can get by with just local civil time and an OS-set timezone. It’s much less common that they need to worry about leap seconds, etc. And many also don’t care about millisecond granularity, etc. If your app does care about all that, however, things become a mess quite quickly.

zoneinfo does in practice hold the historical info before 1970 when it can do so easily in its framework: https://en.wikipedia.org/wiki/UTC%2B01:24

  % zdump -i Europe/Warsaw | head
  
  TZ="Europe/Warsaw"
  - - +0124 LMT
  1880-01-01 00 +0124 WMT
  1915-08-04 23:36 +01 CET
  1916-05-01 00 +02 CEST 1
  1916-10-01 00 +01 CET
  1917-04-16 03 +02 CEST 1
  1917-09-17 02 +01 CET
  1918-04-15 03 +02 CEST 1
  % zdump -i Europe/Kaliningrad | head -20
  
  TZ="Europe/Kaliningrad"
  - - +0122 LMT
  1893-03-31 23:38 +01 CET
  1916-05-01 00 +02 CEST 1
  1916-10-01 00 +01 CET
  1917-04-16 03 +02 CEST 1
  1917-09-17 02 +01 CET
  1918-04-15 03 +02 CEST 1
  1918-09-16 02 +01 CET
  1940-04-01 03 +02 CEST 1
  1942-11-02 02 +01 CET
  1943-03-29 03 +02 CEST 1
  1943-10-04 02 +01 CET
  1944-04-03 03 +02 CEST 1
  1944-10-02 02 +01 CET
  1945-04-02 03 +02 CEST 1
  1945-04-10 00 +02 EET
  1945-04-29 01 +03 EEST 1
  1945-10-31 23 +02 EET
  %

It is, there are a couple of timezones where not only there is a hour difference, but even a 30 and 45 minutes difference. India is UTC +5:30, and Lord Howe Island is UTC +10:30 / +11:00 and New Zealand, Chatham Islands is UTC +12:45 / +13:45, Iran is UTC +3:30 / +4:30 and so on. Where the format is X / Y, that means X is Standard Time, and Y is Daylight time.

Messy.

I think the full list can be found here: https://www.timeanddate.com/time/time-zones-interesting.html

You can use a Bash script that can give you an exhaustive list based on files from /usr/share/zoneinfo/, i.e. find timezones with non-whole hour offsets.

I don’t understand this. What practical difference does it make making the time to round to the nearest quarter of an hour instead of the nearest hour? Personally, I don’t care if noon (sun is in zenith) happens half an hour before 12:00 or half an hour after.

Why do such time zones exist?

Well, I do not know the answer to that, my guess is that it is for historical, political, geographical, and socio-economic reasons.

For example in terms of India, they had two timezones before they adopted a compromise: UTC+5:30.

Nepal uses UTC+5:45, partly to distinguish itself from Indian Standard Time, reinforcing national identity.

Even worse, there are some areas where the timezone depends on your religion. Lebanon had "Muslim time" and "Christian time" at one point (Unsure if that's still a thing)

There's a joke about Northern Ireland, where if you claim to be an atheist, they would ask you, yes but which side - Catholic or Protestant?

The worst is probably Marocco DST

https://en.wikipedia.org/wiki/Daylight_saving_time_in_Morocc...

Calculating Ramadan is something software packages most likely won't do. To get the visibility of the moon correct, I think you have to know the exact location - and maybe check, if there is a mountain obstructing the view.

> India, they had two timezones before they adopted a compromise: UTC+5:30.

Truly, a compromise is when nobody is happy. ._\

Koenigsberg was conquered by the Soviets in April 1945, but the final Soviet-Polish border was only established in August of the same year. I wonder when the official switch to EET was made. For several months, the future of the city was a bit uncertain.

Truthfully, I do not know the story behind it. If you do, feel free to share.

Oh by the way, check this out, this is one of the news in 2025b tzdata:

  # From Roozbeh Pournader (2025-03-18):
  # ... the exact time of Iran's transition from +0400 to +0330 ... was Friday
  # 1357/8/19 AP=1978-11-10. Here's a newspaper clip from the Ettela'at
  # newspaper, dated 1357/8/14 AP=1978-11-05, translated from Persian
  # (at https://w.wiki/DUEY):
  # Following the government's decision about returning the official time
  # to the previous status, the spokesperson for the Ministry of Energy
  # announced today: At the hour 24 of Friday 19th of Aban (=1978-11-10),
  # the country's time will be pulled back half an hour.

From https://github.com/eggert/tz/blob/main/asia#L1503.

Pretty sure we can find a lot more oddities that are way worse.