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LibJS/Date: Ensure YearFromTime(t) holds invariant after approximation

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As of https://tc39.es/ecma262/#sec-yearfromtime, YearFromTime(t) should
return `y` such that `TimeFromYear(YearFromTime(t)) <= t`. This wasn't
held, since the approximation contained decimal digits that would nudge
the final value in the wrong direction.

Adapted from Kiesel:
6548a85743

Co-authored-by: Linus Groh <mail@linusgroh.de>
This commit is contained in:
Jesús (gsus) Lapastora 2023-10-22 22:01:38 +02:00 • committed by Tim Flynn
parent 5870a1a9a1
commit 2086b8df9c
2 changed files with 10 additions and 1 deletions
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2
Userland/Libraries/LibJS/Runtime/Date.cpp
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@ -138,7 +138,7 @@ i32 year_from_time(double t)
return NumericLimits<i32>::max();
// Approximation using average number of milliseconds per year. We might have to adjust this guess afterwards.
auto year = static_cast<i32>(t / (365.2425 * ms_per_day) + 1970);
auto year = static_cast<i32>(floor(t / (365.2425 * ms_per_day) + 1970));
auto year_t = time_from_year(year);
if (year_t > t)