date input: negative fractional epoch (`@-1.5`) truncates instead of floors

[sylvestre]

Summary

GNU date floors negative fractional epoch inputs (rounds toward negative infinity): @-1.5 → -2. parse_datetime truncates toward zero: @-1.5 → -1. The same off-by-one shows up on any negative fractional @N input.

Found by a fuzz_date run on uutils/coreutils.

Reproduction

$ LC_ALL=C TZ=UTC /usr/bin/date -d '@-1.5' '+%s'
-2

$ LC_ALL=C TZ=UTC target/debug/coreutils date -d '@-1.5' '+%s'
-1

$ LC_ALL=C TZ=UTC /usr/bin/date -d '@-893375784.554767216' '+%s'
-893375785

$ LC_ALL=C TZ=UTC target/debug/coreutils date -d '@-893375784.554767216' '+%s'
-893375784

Positive fractional epochs agree between the two.

Test (in uutils/coreutils integration suite)

#[test]
fn test_date_negative_fractional_epoch_flooring() {
    new_ucmd!()
        .env("LC_ALL", "C")
        .env("TZ", "UTC")
        .arg("-d")
        .arg("@-1.5")
        .arg("+%s")
        .succeeds()
        .stdout_is("-2\n");
}