Timestamps & Timekeeping
Discontinuities in civil time Curiosity
Timestamps that never happened
Not every UTC timestamp represents a time which actually happened.
For example, the 0.1 second period from 1968-01-31 23:59:59.9 to 1968-02-01 00:00:00.0 never happened. An adjustment to the TAI-UTC relationship created a discontinuity that skipped right over that 0.1s period.
These are essentially reverse-leap-seconds, though none of them have been more than a fraction of a second since UTC started being tracked. There haven’t been any of these since 1968, and there most likely won’t be any for a long time.
Timestamps that happened twice
UTC itself defines a leap second as inserting an additional second after 23:59:59: 23:59:60.
However, computer systems represent leap seconds in different ways. A linux machine will set the clock back by 1 second at the end of 23:59:59, effectively repeating that second.
Therefore, on computer systems there are some UTC timestamps that are ambiguous and represent two different moments in actual time.
TAI-UTC relationship
The relationship between TAI (actual, physical atomic clock time) and UTC is defined here: https://hpiers.obspm.fr/eop-pc/earthor/utc/TAI-UTC_tab.html
Alternative Calendars Curiosity
Symmetry454
Symmetry454 is a particularly elegant alternative calendar.
Every month is an integer number of weeks and always start on Monday. Leap years add a whole leap week to keep the year an integer number of weeks.
https://en.wikipedia.org/wiki/Symmetry454
Zero-Indexing the Gregorian Calendar
In this blog post, Qntm imagines a cleverly backwards-compatible version of the traditional Gregorian calendar in which the first day of the year is 20XX-00-00.