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Comment by throwaway_12629 | Hacker News Reader

Technically, you're not likely to to have to fix a DST bug at 3AM any day but Sunday.

That's a great example of the kind of wrong assumption that makes dealing with dates and times so challenging.

Some countries start on a Friday or Saturday and until 2022 Iran could start any day of the week although never at 3AM.

    // call foo() one day from now:
    sleep(86400); foo();

Sorry, sleep returned a Promise and you didn't await it. You called foo() immediately.

What you want there is to stop saying "day" and instead say "24 hours." This way the code is correct and you don't need to deal with time weirdness.

No, because if I want something to happen everyday at 12 o'clock, I have to wait for one day, if I wait for 24 hours, I will be off by an hour for half of the year.

Only if you live in one of the brain dead countries that observe the dst anachronism.

ok, but they still observe it and you still have to deal with it in your code.

our personal convictions don't change that fact.

Why do you want something to happen everyday at 12 o'clock specifically? If this is truly what you want, sure.

Lunch break reminder popup?

You seem to assume that a day always has 24 hours. Common (but not only) non-24h day lengths are: - 23 hours - 25 hours - 24 hours 1 second - 23 hours 59 minutes 59 seconds

You could assume that a day isn't exactly 24 hours, but it's close-ish to 24 hours. Nope, not even close.

And that assumes that we can treat an hour as a precise measure of time (we can't). On some systems, even a second is not a precise measure of time (second smearing).

To make things worse, those are "simple" edge cases.

Time is hard. I'm not sure if I can make any statement about time that is true.

I am saying that you shouldn't use day as a unit of time. You should use second, minute, hour, etc, because these have a constant duration. sleep(86400) should reliably make your thread sleep for at least 24 hours.

It depends on the context and the system you’re working with. In some systems, an hour may last 3599, 3600 or 3601 seconds (due to the leap second), a minute may be 59,60 or 61 seconds. Even a second is not always a „true” second.

There’s no single time unit that works for all situations.

Some countries alter their observance of DST in line with their observance of Ramadan, which means that the time-offset changes aligned with Ramadan.

Ramadan is observed from one visual sighting of a crescent moon to the next.

Cloud conditions may prevent sighting and thereby alter the official start of Ramadan for an individual location, and from time-to-time, the start of a country's change in timezone.

Cue a longish article titled 'Falsehoods Programmers Believe About Time'

> Some countries alter their observance of DST in line with their observance of Ramadan, which means that the time-offset changes aligned with Ramadan.

Only Morocco does this, I believe, and it's not even clear that that's actually official time at this point. In 2018, Morocco abolished DST, but it seems unclear what that means in practice.

I'd love it if someone from Morocco could weigh in on what the actual situation is on the ground. Do people still change their clocks for Ramadan? Would they be annoyed if a website kept Moroccan users on standard time during Ramadan?

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