fix(relative): add the fraction of a negative relative second count

src/items/relative.rs

@@ -59,9 +59,23 @@ impl TryFrom<Relative> for jiff::Span {
             Relative::Days(days) => jiff::Span::new().try_days(days),
             Relative::Hours(hours) => jiff::Span::new().try_hours(hours),
             Relative::Minutes(minutes) => jiff::Span::new().try_minutes(minutes),
-            Relative::Seconds(seconds, nanoseconds) => jiff::Span::new()
-                .try_seconds(seconds)
-                .and_then(|span| span.try_nanoseconds(nanoseconds)),
+            Relative::Seconds(seconds, nanoseconds) => {
+                // `Relative::Seconds` is a floor decomposition: the value is
+                // `seconds + nanoseconds / 1e9` with a non-negative fraction, so
+                // -0.25 seconds is held as `(-1, 750_000_000)`. A `jiff::Span`
+                // is sign-uniform, so the fraction has to be rebalanced onto the
+                // sign of the whole seconds first; combining the fields as they
+                // are would subtract the fraction instead of adding it.
+                let (seconds, nanoseconds) = if seconds < 0 && nanoseconds > 0 {
+                    (seconds + 1, -i64::from(1_000_000_000 - nanoseconds))
+                } else {
+                    (seconds, i64::from(nanoseconds))
+                };
+
+                jiff::Span::new()
+                    .try_seconds(seconds)
+                    .and_then(|span| span.try_nanoseconds(nanoseconds))
+            }
         }
         .map_err(|_| "relative value is invalid")
     }

tests/time.rs

@@ -280,3 +280,39 @@ fn test_time_seconds_ago_invalid(#[case] input: &str) {
         "Input string '{input}' did not produce an error when parsing"
     );
 }
+
+// Fractional relative seconds, checked against GNU date 9.11 with
+// TZ=UTC date --date="2026-08-27 12:00:00 <input>" +"%H:%M:%S.%N"
+#[rstest]
+#[case::plus_half("+0.5 sec", "12:00:00.500000000")]
+#[case::minus_half("-0.5 sec", "11:59:59.500000000")]
+#[case::plus_one_and_a_half("+1.5 sec", "12:00:01.500000000")]
+#[case::minus_one_and_a_half("-1.5 sec", "11:59:58.500000000")]
+#[case::plus_quarter("+0.25 sec", "12:00:00.250000000")]
+#[case::minus_quarter("-0.25 sec", "11:59:59.750000000")]
+#[case::plus_one_and_three_quarters("+1.75 sec", "12:00:01.750000000")]
+#[case::minus_one_and_three_quarters("-1.75 sec", "11:59:58.250000000")]
+#[case::plus_two_and_an_eighth("+2.125 sec", "12:00:02.125000000")]
+#[case::minus_two_and_an_eighth("-2.125 sec", "11:59:57.875000000")]
+#[case::minus_nine_nanoseconds("-0.000000009 sec", "11:59:59.999999991")]
+#[case::minus_almost_one("-0.999999999 sec", "11:59:59.000000001")]
+#[case::plus_half_ago("+0.5 sec ago", "11:59:59.500000000")]
+#[case::minus_half_ago("-0.5 sec ago", "12:00:00.500000000")]
+#[case::plus_one_and_a_half_ago("+1.5 sec ago", "11:59:58.500000000")]
+#[case::unsigned_half_ago("0.5 sec ago", "11:59:59.500000000")]
+#[case::unsigned_half("0.5 sec", "12:00:00.500000000")]
+#[case::plus_zero_fraction("+0.0 sec", "12:00:00.000000000")]
+#[case::minus_zero_fraction("-0.0 sec", "12:00:00.000000000")]
+#[case::minus_whole_with_fraction_zero("-1.0 sec", "11:59:59.000000000")]
+#[case::minus_whole("-1 second ago", "12:00:01.000000000")]
+#[case::plural_minus_half("-0.5 seconds", "11:59:59.500000000")]
+#[case::abbreviated_minus_half("-0.5 secs", "11:59:59.500000000")]
+fn test_relative_fractional_seconds(#[case] input: &str, #[case] expected: &str) {
+    let base = "2026-08-27 12:00:00"
+        .parse::<DateTime>()
+        .unwrap()
+        .to_zoned(TimeZone::UTC)
+        .unwrap();
+
+    check_time(input, expected, "%H:%M:%S.%N", Some(base));
+}