diff --git a/src/app/api/measurements/batch/__tests__/aggregated-hr-wire.test.ts b/src/app/api/measurements/batch/__tests__/aggregated-hr-wire.test.ts
index 336f9c64d..be4f79dd3 100644
--- a/src/app/api/measurements/batch/__tests__/aggregated-hr-wire.test.ts
+++ b/src/app/api/measurements/batch/__tests__/aggregated-hr-wire.test.ts
@@ -172,6 +172,39 @@ describe("POST /api/measurements/batch — aggregated HR wire contract (iOS #34)
     expect(rows[0].valueMax).toBe(96);
   });
 
+  it.each([
+    // out-of-plausible-range spread (a sensor glitch / spurious discreteMax)
+    { valueMin: 58, valueMax: 99999 },
+    { valueMin: -9999, valueMax: 96 },
+    // mis-ordered: valueMin above the average, or valueMax below it
+    { valueMin: 80, valueMax: 96 },
+    { valueMin: 58, valueMax: 70 },
+  ])(
+    "v1.30.8 — drops an out-of-range or mis-ordered spread to null but keeps the average %o",
+    async (spread) => {
+      vi.mocked(prisma.measurement.createMany).mockResolvedValue({ count: 1 });
+      const res = await POST(
+        makeRequest({ entries: [hrBucketEntry(HR_BUCKET_ID, 72, spread)] }),
+      );
+      expect(res.status).toBe(200);
+      const createArg = vi.mocked(prisma.measurement.createMany).mock
+        .calls[0][0];
+      const rows = (
+        createArg as {
+          data: {
+            value: number;
+            valueMin: number | null;
+            valueMax: number | null;
+          }[];
+        }
+      ).data;
+      // The trustworthy average survives; the invalid spread is dropped.
+      expect(rows[0].value).toBe(72);
+      expect(rows[0].valueMin).toBeNull();
+      expect(rows[0].valueMax).toBeNull();
+    },
+  );
+
   it("overwrites min/max alongside the average on a re-post", async () => {
     vi.mocked(prisma.measurement.findMany).mockResolvedValue([
       { type: "PULSE", source: "APPLE_HEALTH", externalId: HR_BUCKET_ID },
diff --git a/src/app/api/measurements/batch/route.ts b/src/app/api/measurements/batch/route.ts
index 33cfe5c94..1bdce805b 100644
--- a/src/app/api/measurements/batch/route.ts
+++ b/src/app/api/measurements/batch/route.ts
@@ -318,15 +318,25 @@ async function postBatch(request: NextRequest): Promise<Response> {
     // per-day cumulative `stats:` total, or a manual entry never carries a
     // spread, so we pin them to null there even if a client mis-sends the
     // fields — `value` stays the single source of truth for those rows.
+    //
+    // v1.30.8 — validate the spread the same way `value` is guarded (above):
+    // it now drives the DAY min/max band + the intraday envelope, so an
+    // out-of-range or mis-ordered spread (a spurious `discreteMin` of 0, a
+    // sensor glitch) would poison the aggregate exactly like a bogus `value`
+    // would. Persist the spread ONLY when both extremes are present, in range,
+    // and correctly ordered around `value`; otherwise drop to null and keep the
+    // trustworthy average. Partial or invalid spread → avg-only, still valid.
     const isHrBucket = isAggregatedBucketExternalId(entry.externalId);
-    const valueMin =
-      isHrBucket && typeof mapped.valueMin === "number"
-        ? mapped.valueMin
-        : null;
-    const valueMax =
-      isHrBucket && typeof mapped.valueMax === "number"
-        ? mapped.valueMax
-        : null;
+    const spreadValid =
+      isHrBucket &&
+      typeof mapped.valueMin === "number" &&
+      typeof mapped.valueMax === "number" &&
+      validateMeasurementRange(mapped.type, mapped.valueMin) === null &&
+      validateMeasurementRange(mapped.type, mapped.valueMax) === null &&
+      mapped.valueMin <= mapped.value &&
+      mapped.value <= mapped.valueMax;
+    const valueMin = spreadValid ? mapped.valueMin : null;
+    const valueMax = spreadValid ? mapped.valueMax : null;
 
     prepared.push({
       index,
diff --git a/src/lib/measurements/__tests__/aggregated-hr-bucket.test.ts b/src/lib/measurements/__tests__/aggregated-hr-bucket.test.ts
index 576e9b453..e07539fcf 100644
--- a/src/lib/measurements/__tests__/aggregated-hr-bucket.test.ts
+++ b/src/lib/measurements/__tests__/aggregated-hr-bucket.test.ts
@@ -90,6 +90,19 @@ describe("isAggregatedBucketExternalId", () => {
   ])("returns false for a non-bucket externalId %s", (id) => {
     expect(isAggregatedBucketExternalId(id as string | null)).toBe(false);
   });
+
+  it.each([
+    // v1.30.8 — these PARSE via V8's silent rollover but are NOT canonical, so
+    // they'd key a row the canonical (toISOString) client never emits → an
+    // un-mergeable duplicate + wrong-day placement. Must be rejected.
+    "stats:HKQuantityTypeIdentifierHeartRate:2026-07-18T24:00:00.000Z", // → 07-19T00
+    "stats:HKQuantityTypeIdentifierHeartRate:2026-04-31T14:30:00.000Z", // → 05-01
+    "stats:HKQuantityTypeIdentifierHeartRate:2026-02-29T14:30:00.000Z", // non-leap → 03-01
+    "stats:HKQuantityTypeIdentifierHeartRate:2026-12-31T24:00:00.000Z", // → 2027-01-01
+  ])("rejects a non-canonical rollover instant %s", (id) => {
+    expect(isAggregatedBucketExternalId(id)).toBe(false);
+    expect(parseAggregatedBucketStart(id)).toBeNull();
+  });
 });
 
 describe("targetsAggregatedBucket", () => {
diff --git a/src/lib/measurements/apple-health-mapping.ts b/src/lib/measurements/apple-health-mapping.ts
index 74d182ae6..e665b18bd 100644
--- a/src/lib/measurements/apple-health-mapping.ts
+++ b/src/lib/measurements/apple-health-mapping.ts
@@ -897,7 +897,14 @@ export function isAggregatedBucketExternalId(
     const suffix = externalId.slice(prefix.length);
     if (!AGGREGATED_BUCKET_SUFFIX_RE.test(suffix)) return false;
     const parsed = new Date(suffix);
-    return !Number.isNaN(parsed.getTime());
+    if (Number.isNaN(parsed.getTime())) return false;
+    // Reject a non-canonical instant that PARSES via V8's silent rollover
+    // (`T24:00`, `2026-04-31`, a non-leap `02-29`) — it would key a row under
+    // a literal string the canonical client (`heartRateBucketExternalId`, which
+    // mints via `toISOString()`) never emits, so the two would never collapse
+    // on the overwrite key. Round-trip equality pins the suffix to the one
+    // canonical form.
+    return parsed.toISOString() === suffix;
   }
   return false;
 }
@@ -937,7 +944,10 @@ export function parseAggregatedBucketStart(
     const suffix = externalId.slice(prefix.length);
     if (!AGGREGATED_BUCKET_SUFFIX_RE.test(suffix)) return null;
     const parsed = new Date(suffix);
-    return Number.isNaN(parsed.getTime()) ? null : parsed;
+    if (Number.isNaN(parsed.getTime())) return null;
+    // Canonical-form gate (mirrors `isAggregatedBucketExternalId`): a rolled-over
+    // instant would place the bucket on the wrong local day.
+    return parsed.toISOString() === suffix ? parsed : null;
   }
   return null;
 }