35. Two timezones that differ, will differ by an integer number of hours.
N+1. OK, historical oddities aside, the offsets between two time zones won't change in the future.
N+2. Changes in the offsets between time zones will occur with plenty of advance notice.
N+3. Daylight savings time happens at the same time every year.
N+4. Daylight savings time happens at the same time in every time zone.
N+5. Daylight savings time always adjusts by an hour.
N+6. Months have either 28, 29, 30, or 31 days.
N+7. The day of the month always advances contiguously from N to either N+1 or 1, with no discontinuities.
Explanations:
(N)-(N+2): There exist enough jurisdictions in the world that time zone changes occur often enough to require regular updates to the time zone database, more frequently than many distribution release schedules occur.
(N+3)-(N+5): Jurisdictions change DST policies even more frequently than they change time zones.
(N+6)-(N+7): September 1752 had 19 days: 1, 2, 14, 15, ..., 29, 30.
In the British Empire, anyway.
As for "all modern computer systems", Unix time is completely ignorant about anything except seconds.
In hindsight, it seems like a brilliant decision.
https://en.wikipedia.org/wiki/POSIX_time#Encoding_time_as_a_...
That's not correct, actually: UNIX time tracks UTC instead of TAI meaning it "corrects" for leap seconds. As a result, UNIX time is not "the number of seconds since epoch" but "86400 * (number of whole days since epoch) + (number of seconds since midnight)", and UNIX time will go forwards (never so far) and backwards on leap seconds (a second will repeat in most implementations as the day goes from 23:59:60 to 00:00:00, as they have the same timestamp).
That's only in the British Empire. Other countries moved to the Gregorian Calendar at different times. It began being adopted on 15 October 1582 (the day after 4 October 1582) in some Roman Catholic countries (Spain, Portugal, Italy, Poland). Russia didn't switch until 1918.
So, you could also have:
N+8. There is only one calendar system in use at one time.
And the list hasn't actually covered the Gregorian calendar at all, since many people may not know the actual leap year rule.
N+9. There is a leap year every year divisible by 4.
The actual rule is that there is a leap year every year divisible by 4, except those divisible by 100, with a further exception to that exception for years divisible by 400, (so there is a leap year on such years). The year 2000 is one such year divisible by 400, so we have not yet passed a year divisible by 4 which is not a leap year since the invention of computers.
> 19. The system clock will never be set to a time that is in the distant past or the far future.
> 20. Time has no beginning and no end.
The reason this is impossible on Windows and downright awkward on Unix is because DST changes from time to time. Unless you have something like tzinfo, you cannot work out a particular date and time by merely adding a duration to another date and time in the past.
Instead, you must work out the timezone you are doing the calculation in, then work out whether the duration needs to incorporate a DST change, which involves working out what that DST changeover date and time was...
Of course, it's impossible on Windows because Microsoft only give the start and end datetime offsets for the current year. Amazingly, after all the patches they've had to issue to fix DST changes over the years, they still haven't implemented a Windows equivalent of tzinfo yet. And may never do so, even though they really aught to, given they are one of the leaders in calendaring software.
37. Okay, quarter hours.
38. Okay, seconds, but it will be a consistent difference if we ignore DST.
40. You can wait for the clock to reach exactly HH:MM:SS by sampling once a second.
The best part was that this was in Javascript (and obviously he was not using a timeout). The entire page would lock up while it waited for the second to elapse. He never even figured out the obvious usability concern because he was so confounded by the fact that the procedure wasn't getting called after a second had passed.
Ok, to be fair, it was a freshman year programming class - not an unexpected or even unusual mistake. But it gave me a chuckle just now remembering it.
Another amusing anecdote: I got an old PowerBook out of storage and booted it up. The battery had run flat, so the clock reset to 1970. Spotlight noticed this and decided it had to reindex the entire drive. At some point during all of this, NTP kicked in and reset the clock to the correct year. I wanted to see how long Spotlight would take to finish indexing, so I popped down the menu. The progress bar indicated that it was about halfway done, and based on how long it had taken so far, it estimated that only another 40 years would be required to finish!
I just found out today about Calcutta time, which until 1948 was GMT+05:30:21, but I didn ’t realize it could be even worse than that.
http://www.nytimes.com/2011/12/30/world/asia/samoa-to-skip-f...
"Aircraft software can be serious business" http://www.defenseindustrydaily.com/f22-squadron-shot-down-b...
"The [F22 Raptor IDL] post-incident report" http://it.slashdot.org/comments.pl?sid=224098&cid=181501...
"[undetected software bug in the] Northrop F-20 Tigershark laser inertial navigation system" http://www.f20a.com/f20ins.htm