View source for On Time, and Why It's Hard

'''The epoch.''' Machines count seconds from January 1, 1970, 00:00:00 UTC, and everything since is an incrementing integer — Unix time.<ref>[https://en.wikipedia.org/wiki/Unix_time Unix time — Wikipedia]</ref> ISO 8601 is the closest thing to a universal human-readable format for the same information — see GAMAYUN's own Date Stamp Recon tool for the actual converter.

'''The Year 2038 Problem.''' A lot of systems store that Unix timestamp as a signed 32-bit integer. That format runs out of road on January 19, 2038, at 03:14:07 UTC — one second later, the count wraps back around to a large negative number, which most software reads as December 1901.<ref>[https://en.wikipedia.org/wiki/Year_2038_problem Year 2038 problem — Wikipedia]</ref> Same failure mode as the Y2K panic, different root cause: Y2K was a storage-shortcut problem (two-digit years), this is an arithmetic-overflow problem (a fixed-width integer running out of bits). Most modern 64-bit systems already sidestepped it. Plenty of embedded and legacy 32-bit systems have not, and won't notice until they do.

'''Timezones ruin everything.''' They aren't clean hour-wide slices — India runs UTC+5:30, Nepal runs UTC+5:45, and some Pacific islands hop entire calendar days depending which side of the International Date Line their government prefers this decade.<ref>[https://en.wikipedia.org/wiki/UTC%2B05:45 UTC+05:45 (Nepal Time) — Wikipedia]</ref> Add daylight saving time (which not every country observes, and which some countries have started and then abandoned more than once) and "what time is it in the other office" stops being arithmetic and starts being a lookup against a constantly-revised political database.

'''Leap seconds.''' Earth's rotation is slowing down, almost imperceptibly, so every so often an official leap second gets inserted to keep clock time lined up with the actual planet.<ref>[https://en.wikipedia.org/wiki/Leap_second Leap second — Wikipedia]</ref> In 2012, a leap second exposed a bug in the Linux kernel's futex implementation that pushed CPU usage on affected servers to 100% — Reddit, Mozilla, and Yelp all had a bad night over one extra second.<ref>[https://www.wired.com/2012/07/leap-second-bug-wreaks-havoc-with-java-linux/ How a Leap Second Wreaked Havoc — Wired, 2012]</ref> Several major tech companies now "smear" the leap second across an entire day of very slightly slower ticks instead of inserting it all at once, specifically so this doesn't happen again. The bugs got annoying enough, often enough, that on November 18, 2022, the world's governments voted at the General Conference on Weights and Measures to retire the leap second entirely by 2035 — after which UTC will simply be allowed to drift out of sync with the Earth's actual rotation rather than being corrected.<ref>[https://www.nist.gov/news-events/news/2022/11/no-more-leap-seconds-until-least-2035 No More Leap Seconds Until at Least 2035 — NIST, 2022]</ref> Russia was the one notable vote against it, reportedly over concerns about its own satellite navigation systems' dependence on the current scheme.

'''How your computer's clock actually stays right.''' Left alone, a computer's internal clock drifts — cheap quartz oscillators gain or lose real seconds per day. NTP (Network Time Protocol), designed by David L. Mills starting in the 1980s, is the quiet background process that fixes this: your machine periodically asks a server it trusts what time it actually is and nudges its own clock toward that answer, smoothly rather than jumping.<ref>[https://en.wikipedia.org/wiki/Network_Time_Protocol Network Time Protocol — Wikipedia]</ref> Those servers form a hierarchy of "strata" — stratum 0 is an actual physical reference clock (atomic or GPS-synchronized), stratum 1 servers are directly connected to one of those, and everything else is a few hops further down the chain.

'''Why cron starts the week on Sunday.''' Cron's day-of-week field runs 0–6 with Sunday as 0, not Monday. It's inherited from POSIX C's <code>struct tm</code>, where <code>tm_wday</code> has always used the same 0-for-Sunday convention, which itself traces back to older US calendar conventions treating Sunday as the start of the week. (ISO 8601 goes the other way entirely for its own week-numbering system — Monday is day 1, and "week 1" of a year is defined as whichever week contains that year's first Thursday.)<ref>[https://en.wikipedia.org/wiki/ISO_8601#Week_dates ISO week date — Wikipedia]</ref>

== See also ==
* [[Calendars Are Political]]
* [[The Odds a UUID Ever Repeats]] — a different kind of "the numbers say this basically never happens"
* [[Browser Fingerprinting]] — another article built on bits of entropy, just applied to identity instead of timestamps

[[Category:Physics & Perception]]