63 lines
3.1 KiB
Markdown
63 lines
3.1 KiB
Markdown
- Timezones
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- They are not always on the hour
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- Difference time offset and timezone
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- (locale stuff, formatting, DST transition dates, etc.)
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- Daylight savings
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- Days can be 23 or 25 hours long
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- Countries change their DST rules all the time
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- Dates, times, datetimes
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- Maybe you don't actually want to store full timestamps for literally everything
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- Time math is tricky
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- Adding 24 hours != adding one day (leap days / seconds)
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- Calendars
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- Historical calendars are very weird (kings just removing or adding days when they felt like it)
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- The calendar we use, and its extensions: Gregorian calendar, proleptic Gregorian calendar
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- Other calendars: Jewish calendar, Chinese lunar calendar (?), Hijiri
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- Primarily used for establishing holidays, but not really for common use any more (with the exception of China...?)
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- Computer clocks
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- CPU time vs. wall time
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- Monotonic clocks
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- Two kinds on Linux: one that smears forward, one that does not
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- Jiffies
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- Atomic clocks
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- NTP / chrony
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- Useful specs:
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- RFC3339: https://datatracker.ietf.org/doc/html/rfc3339
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- iCal: https://icalendar.org/RFC-Specifications/iCalendar-RFC-5545/
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- Recurrence rules (RRULE): https://icalendar.org/iCalendar-RFC-5545/3-8-5-3-recurrence-rule.html
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- NOBODY USES ISO8601 STOP SAYING THAT THEY DO
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- NO ONE KNOWS WHAT IT IS
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- RFC3339 IS THE ONLY ONE ANYONE SHOULD USE
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- Accuracy of onboard clocks
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- Quartz oscillators - subject to drift
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- How to correct inaccurate clocks
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- Time smearing
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- Presentation and formatting?
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- The UNIX epoch, and the year 2038 problem
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- ISO "week date" (does this matter to anyone)
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- Falsehoods I think are actually important
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- "Always store time in UTC"
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- "Always store fully-qualified timestamps" i.e. datetimes with timezone, not just dates
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- "The smallest unit of time is X" (i.e. store the start and end of your time ranges precisely, never store 11:59:59)
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You get a time you trust - then you work with it in some way (???)
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- https://infiniteundo.com/post/25326999628/falsehoods-programmers-believe-about-time
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- https://engineering.fb.com/2021/08/11/open-source/time-appliance/
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- https://github.com/opencomputeproject/Time-Appliance-Project/tree/master/Open-Time-Server/
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- https://lettier.github.io/posts/2016-04-26-lets-make-a-ntp-client-in-c.html
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- https://audiogramii.wordpress.com/2018/03/31/time-smearing/
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- https://docs.ntpsec.org/latest/leapsmear.html
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- https://aws.amazon.com/blogs/aws/look-before-you-leap-the-coming-leap-second-and-aws/
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- https://sudonull.com/post/29903-Writing-a-Simple-NTP-Client
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- https://www.oreilly.com/library/view/linux-device-drivers/9781785280009/4041820a-bbe4-4502-8ef9-d1913e133332.xhtml
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- https://www.youtube.com/watch?v=-5wpm-gesOY
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- Official timezone database: https://www.iana.org/time-zones
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## How to use dates and times
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## Time accuracy / how to get accurate times
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## Fun tangents
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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. |