diff --git a/src/scoring/preview.ts b/src/scoring/preview.ts
index f6d8ca6281..3ba286e04f 100644
--- a/src/scoring/preview.ts
+++ b/src/scoring/preview.ts
@@ -1199,13 +1199,19 @@ function constant(constants: Record<string, number>, key: string): number {
  * Resolve a repo's time-decay curve: each parameter is the repo's per-repo override (from the registry's
  * `scoring.time_decay`) when present, else the global default constant from the live scoring snapshot.
  * Mirrors upstream's `resolve_time_decay` (RepoTimeDecayConfig overlaid on the module constants).
+ *
+ * Parity note (#1320): upstream coerces ONLY `grace_period_hours` to an integer
+ * (`grace_period_hours=int(pick(...))`) while the three curve params stay floats. A maintainer may legally
+ * configure a fractional grace (upstream validates `0 <= grace_period_hours <= 168`), so the resolved grace
+ * must be truncated toward zero to match the validator — otherwise a PR aged between `trunc(grace)` and
+ * `grace` is treated as fresh in the preview but already decaying upstream.
  */
 export function resolveTimeDecay(
   constants: Record<string, number>,
   overrides?: RepoTimeDecayOverrides | null,
 ): { gracePeriodHours: number; sigmoidMidpointDays: number; sigmoidSteepness: number; minMultiplier: number } {
   return {
-    gracePeriodHours: pickOverride(overrides?.gracePeriodHours, constant(constants, "TIME_DECAY_GRACE_PERIOD_HOURS")),
+    gracePeriodHours: Math.trunc(pickOverride(overrides?.gracePeriodHours, constant(constants, "TIME_DECAY_GRACE_PERIOD_HOURS"))),
     sigmoidMidpointDays: pickOverride(overrides?.sigmoidMidpointDays, constant(constants, "TIME_DECAY_SIGMOID_MIDPOINT")),
     sigmoidSteepness: pickOverride(overrides?.sigmoidSteepness, constant(constants, "TIME_DECAY_SIGMOID_STEEPNESS_SCALAR")),
     minMultiplier: pickOverride(overrides?.minMultiplier, constant(constants, "TIME_DECAY_MIN_MULTIPLIER")),
diff --git a/test/unit/scoring.test.ts b/test/unit/scoring.test.ts
index 7166626a11..1ee4a5b24c 100644
--- a/test/unit/scoring.test.ts
+++ b/test/unit/scoring.test.ts
@@ -1747,6 +1747,23 @@ NOVELTY_BONUS_SCALAR = 3
       expect(calculateTimeDecay(120, c, { sigmoidMidpointDays: 5 })).toBeCloseTo(0.5, 5);
     });
 
+    it("truncates a fractional grace_period_hours override toward zero, mirroring upstream int() (#1320)", () => {
+      const c = DEFAULT_SCORING_CONSTANTS;
+      // Upstream resolve_time_decay does `grace_period_hours=int(pick(...))` — and only that field. A
+      // fractional override (legal under upstream's 0..168 range check) resolves to its truncated integer,
+      // while the float curve params are untouched.
+      expect(resolveTimeDecay(c, { gracePeriodHours: 13.9 }).gracePeriodHours).toBe(13);
+      expect(resolveTimeDecay(c, { gracePeriodHours: 13.9, sigmoidSteepness: 0.4 })).toEqual({
+        gracePeriodHours: 13,
+        sigmoidMidpointDays: 10,
+        sigmoidSteepness: 0.4,
+        minMultiplier: 0.05,
+      });
+      // The boundary case the bug hid: a PR aged between trunc(grace) and grace is already decaying
+      // upstream (13.5 >= 13), so the preview must decay it too rather than reporting it as fresh.
+      expect(calculateTimeDecay(13.5, c, { gracePeriodHours: 13.9 })).toBeLessThan(1);
+    });
+
     it("applies each live repo's resolved curve in the preview (per-repo, not global)", () => {
       const input: ScorePreviewInput = { repoFullName: repo.fullName, sourceTokenScore: 58, totalTokenScore: 600, sourceLines: 60, openPrCount: 0, credibility: 1, applyTimeDecay: true, prAgeHours: 18 };
       // Repo with a 24h grace override (like JSONbored/gittensory) → an 18h-old PR is still fresh.