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manifoldgeo | 2 years ago

How does something like `let y: Time = 1 year` work? Does it take into consideration the idea of leap years and leap seconds, counting a single year as 365.2422 days[0]? Or does it count as 365 days?

I got curious and installed the CLI tool[1] and found that it does indeed account for leap second / leap years:

>>> let jahr: Time = 1 year

>>> let tage: Time = jahr -> days

>>> tage

    = 365.243 day    [Time]
References:

0: https://museum.seiko.co.jp/en/knowledge/story_01/

1: https://numbat.dev/doc/cli-installation.html

discuss

order

vlovich123|2 years ago

I feel like that’s unit confusion. Converting from year to day should require you to specify what calendar year you’re in to resolve the ambiguity. Otherwise I set an alarm for today + 1 year and things are off by 6 hours.

Time is nasty because there’s lots of overloaded concepts and bugs hide in the implicit conversions between meanings.

I’m also kinda curious what the underlying type is. Is it a double or do they use arbitrary precision math and take the perf hit.

SenAnder|2 years ago

> I feel like that’s unit confusion. Converting from year to day should require you to specify what calendar year you’re in to resolve the ambiguity.

A year has two meanings - a calendar year, with all the leap days and seconds and timezones, or a duration of time. The latter is still useful, e.g. when someone states that Proxima Centauri is 4.2 light-years away, they don't want to deal with leap-days.

Decent time libraries have separate ways to deal with durations and dates.

agalunar|2 years ago

365·243 ought to be 365·2425 exactly:

Per 400 years, there is one leap day every 4 years (100 leap days), except when the year is divisible by 100 (so we overcounted by 4 and there are 100 – 4 = 96 leap days), except when the year is divisible by 400 (so we need to add that day back and arrive at 100 – 4 + 1 = 97). This gives us 97/400 = 0·2425.

The tropical year is about 365·24219 days long, but that's not relevant to timekeeping.