[agalunar]

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.

[sharkdp]

> 365·243 ought to be 365·2425 exactly:

Yes. This is also how it is defined: https://github.com/sharkdp/numbat/blob/ba9e97b1fbf6353d24695...

The calculation above is showing a rounded result (6 significant digits by default).

[agalunar]

That's what I figured! but thought the derivation would be fun to share with people reading the comments.