diff --git a/packages/tlock/src/freshness.test.ts b/packages/tlock/src/freshness.test.ts
index 4e803a0..9a3cae1 100644
--- a/packages/tlock/src/freshness.test.ts
+++ b/packages/tlock/src/freshness.test.ts
@@ -2,7 +2,7 @@
 import { test } from "node:test";
 import assert from "node:assert/strict";
 
-import { classifyDrandRound, DEFAULT_STALE_THRESHOLD_MS } from "./freshness.js";
+import { classifyDrandRound, computePublishAtMs, DEFAULT_STALE_THRESHOLD_MS } from "./freshness.js";
 
 test("freshness: missing or invalid round returns unknown", () => {
   const info = { genesis_time: 1677685200, period: 3 };
@@ -76,3 +76,32 @@ test("freshness: stale round", () => {
   // Custom threshold stale
   assert.equal(classifyDrandRound(round, info, 1030011, 10).status, "stale");
 });
+
+test("computePublishAtMs rejects unsafe round and period combinations", () => {
+  const info = { genesis_time: 1_000_000_000, period: 3 };
+
+  assert.equal(computePublishAtMs(info, Number.MAX_SAFE_INTEGER), null);
+  assert.equal(
+    computePublishAtMs({ genesis_time: Number.MAX_SAFE_INTEGER, period: 2 }, 1_000),
+    null,
+  );
+  assert.equal(computePublishAtMs({ genesis_time: 0, period: 3 }, 1), 3000);
+});
+
+test("freshness: unsafe timestamp math returns unknown near MAX_SAFE_INTEGER", () => {
+  const info = { genesis_time: Number.MAX_SAFE_INTEGER - 1, period: 2 };
+  const round = 2;
+  const now = 1_700_000_000_000;
+
+  const res = classifyDrandRound(round, info, now);
+  assert.equal(res.status, "unknown");
+  assert.match(String(res.reason ?? ""), /overflow|unsafe/i);
+});
+
+test("freshness: valid boundary round still classifies correctly", () => {
+  const info = { genesis_time: 1000, period: 1 };
+  const round = 1_000_000;
+  const publishAtMs = computePublishAtMs(info, round);
+  assert.equal(publishAtMs, 1_001_000_000);
+  assert.equal(classifyDrandRound(round, info, publishAtMs!).status, "fresh");
+});
diff --git a/packages/tlock/src/freshness.ts b/packages/tlock/src/freshness.ts
index 3792aba..9536acc 100644
--- a/packages/tlock/src/freshness.ts
+++ b/packages/tlock/src/freshness.ts
@@ -15,6 +15,39 @@ export interface FreshnessResult {
   ageMs?: number;
 }
 
+/**
+ * Derives a Drand round publication timestamp in milliseconds, rejecting
+ * intermediate calculations that would exceed Number.MAX_SAFE_INTEGER.
+ */
+export function computePublishAtMs(info: DrandRoundInfo, round: number): number | null {
+  if (!Number.isSafeInteger(round) || round <= 0) {
+    return null;
+  }
+  if (!Number.isSafeInteger(info.period) || info.period <= 0) {
+    return null;
+  }
+  if (!Number.isSafeInteger(info.genesis_time) || info.genesis_time < 0) {
+    return null;
+  }
+
+  const offsetSeconds = info.period * round;
+  if (!Number.isSafeInteger(offsetSeconds)) {
+    return null;
+  }
+
+  const publishAtSeconds = info.genesis_time + offsetSeconds;
+  if (!Number.isSafeInteger(publishAtSeconds) || publishAtSeconds < 0) {
+    return null;
+  }
+
+  const publishAtMs = publishAtSeconds * 1000;
+  if (!Number.isSafeInteger(publishAtMs)) {
+    return null;
+  }
+
+  return publishAtMs;
+}
+
 /**
  * Classifies a Drand round's freshness deterministically using the current time
  * and Drand network info.
@@ -41,8 +74,10 @@ export function classifyDrandRound(
     return { status: "unknown", reason: "invalid timestamp" };
   }
 
-  // Compute publish time matching the existing keeper logic convention.
-  const publishAtMs = (info.genesis_time + info.period * round) * 1000;
+  const publishAtMs = computePublishAtMs(info, round);
+  if (publishAtMs == null) {
+    return { status: "unknown", reason: "timestamp overflow or unsafe calculation" };
+  }
 
   if (nowMs < publishAtMs) {
     return {