Add 'ideas/time.md'

ideas/time.md

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+- 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
+        - 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:
+    - RFC3339: https://datatracker.ietf.org/doc/html/rfc3339
+    - iCal: https://icalendar.org/RFC-Specifications/iCalendar-RFC-5545/
+    - Recurrence rules (RRULE): https://icalendar.org/iCalendar-RFC-5545/3-8-5-3-recurrence-rule.html
+- 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.