3.3 KiB
3.3 KiB
This will be our article three. Ben will get to this when he has time, unless someone else wants to jump in and lead the content of this.
- Timezones
- They are not always on the hour
- Difference time offset and timezone
- (locale stuff, formatting, DST transition dates, etc.)
- Daylight savings
- Days can be 23 or 25 hours long
- Countries change their DST rules all the time
- Dates, times, datetimes
- Maybe you don't actually want to store full timestamps for literally everything
- Time math is tricky
- Adding 24 hours != adding one day (leap days / seconds)
- Calendars
- Historical calendars are very weird (kings just removing or adding days when they felt like it)
- The calendar we use, and its extensions: Gregorian calendar, proleptic Gregorian calendar
- Other calendars: Jewish calendar, Chinese lunar calendar (?), Hijiri, Japanese calendar (?)
- Primarily used for establishing holidays, but not really for common use any more (with the exception of China...?)
- Computer clocks
- CPU time vs. wall time
- Monotonic clocks
- Two kinds on Linux: one that smears forward, one that does not
- Jiffies
- Atomic clocks
- NTP / chrony
- Useful specs:
- NOBODY USES ISO8601 STOP SAYING THAT THEY DO
- NO ONE KNOWS WHAT IT IS
- RFC3339 IS THE ONLY ONE ANYONE SHOULD USE
- Accuracy of onboard clocks
- Quartz oscillators - subject to drift
- How to correct inaccurate clocks
- Time smearing
- Presentation and formatting?
- The UNIX epoch, and the year 2038 problem
- ISO "week date" (does this matter to anyone)
- Falsehoods I think are actually important
- "Always store time in UTC"
- "Always store fully-qualified timestamps" i.e. datetimes with timezone, not just dates
- "The smallest unit of time is X" (i.e. store the start and end of your time ranges precisely, never store 11:59:59)
You get a time you trust - then you work with it in some way (???)
- https://infiniteundo.com/post/25326999628/falsehoods-programmers-believe-about-time
- https://engineering.fb.com/2021/08/11/open-source/time-appliance/
- https://github.com/opencomputeproject/Time-Appliance-Project/tree/master/Open-Time-Server/
- https://lettier.github.io/posts/2016-04-26-lets-make-a-ntp-client-in-c.html
- https://audiogramii.wordpress.com/2018/03/31/time-smearing/
- https://docs.ntpsec.org/latest/leapsmear.html
- https://aws.amazon.com/blogs/aws/look-before-you-leap-the-coming-leap-second-and-aws/
- https://sudonull.com/post/29903-Writing-a-Simple-NTP-Client
- https://www.oreilly.com/library/view/linux-device-drivers/9781785280009/4041820a-bbe4-4502-8ef9-d1913e133332.xhtml
- https://www.youtube.com/watch?v=-5wpm-gesOY
- Official timezone database: https://www.iana.org/time-zones
How to use dates and times
Time accuracy / how to get accurate times
Fun tangents
East Asian countries treat birthdays differently: https://en.wikipedia.org/wiki/East_Asian_age_reckoning. In particular, Koreans (and others??) all celebrate their birthday on January 1, and start counting at 1 instead of 0.