diff --git a/extensions/slack/src/monitor/monitor.media.test.ts b/extensions/slack/src/monitor/monitor.media.test.ts
index feb9f5871c3..86b6669d3af 100644
--- a/extensions/slack/src/monitor/monitor.media.test.ts
+++ b/extensions/slack/src/monitor/monitor.media.test.ts
@@ -61,6 +61,45 @@ describe("resolveSlackThreadStarter cache", () => {
     expect(replies).toHaveBeenCalledTimes(2);
   });
 
+  it("drops cached thread starters when the current clock is not a valid date timestamp", async () => {
+    const nowSpy = vi.spyOn(Date, "now").mockReturnValue(1_700_000_000_000);
+    const { replies, client } = createThreadStarterRepliesClient();
+
+    const first = await resolveSlackThreadStarter({
+      channelId: "C1",
+      threadTs: "1000.1",
+      client,
+    });
+    nowSpy.mockReturnValue(Number.NaN);
+    const second = await resolveSlackThreadStarter({
+      channelId: "C1",
+      threadTs: "1000.1",
+      client,
+    });
+
+    expect(first).toEqual(second);
+    expect(replies).toHaveBeenCalledTimes(2);
+  });
+
+  it("does not cache thread starters when the expiry timestamp would exceed the valid date range", async () => {
+    vi.spyOn(Date, "now").mockReturnValue(8_640_000_000_000_000);
+    const { replies, client } = createThreadStarterRepliesClient();
+
+    const first = await resolveSlackThreadStarter({
+      channelId: "C1",
+      threadTs: "1000.1",
+      client,
+    });
+    const second = await resolveSlackThreadStarter({
+      channelId: "C1",
+      threadTs: "1000.1",
+      client,
+    });
+
+    expect(first).toEqual(second);
+    expect(replies).toHaveBeenCalledTimes(2);
+  });
+
   it("does not cache empty starter text", async () => {
     const { replies, client } = createThreadStarterRepliesClient({
       messages: [{ text: "   ", user: "U1", ts: "1000.1" }],
diff --git a/extensions/slack/src/monitor/thread.ts b/extensions/slack/src/monitor/thread.ts
index 9425078cb35..794d41cd3d6 100644
--- a/extensions/slack/src/monitor/thread.ts
+++ b/extensions/slack/src/monitor/thread.ts
@@ -1,6 +1,10 @@
 import type { WebClient as SlackWebClient } from "@slack/web-api";
 import { pruneMapToMaxSize } from "openclaw/plugin-sdk/collection-runtime";
 import { formatErrorMessage } from "openclaw/plugin-sdk/error-runtime";
+import {
+  asDateTimestampMs,
+  resolveExpiresAtMsFromDurationMs,
+} from "openclaw/plugin-sdk/number-runtime";
 import { formatSlackFileReferenceList } from "../file-reference.js";
 import type { SlackFile } from "../types.js";
 import { logVerbose } from "./thread.runtime.js";
@@ -15,7 +19,7 @@ export type SlackThreadStarter = {
 
 type SlackThreadStarterCacheEntry = {
   value: SlackThreadStarter;
-  cachedAt: number;
+  expiresAt: number;
 };
 
 const THREAD_STARTER_CACHE = new Map<string, SlackThreadStarterCacheEntry>();
@@ -23,9 +27,13 @@ const THREAD_STARTER_CACHE_TTL_MS = 6 * 60 * 60_000;
 const THREAD_STARTER_CACHE_MAX = 2000;
 
 function evictThreadStarterCache(): void {
-  const now = Date.now();
+  const now = asDateTimestampMs(Date.now());
+  if (now === undefined) {
+    THREAD_STARTER_CACHE.clear();
+    return;
+  }
   for (const [cacheKey, entry] of THREAD_STARTER_CACHE.entries()) {
-    if (now - entry.cachedAt > THREAD_STARTER_CACHE_TTL_MS) {
+    if (asDateTimestampMs(entry.expiresAt) === undefined || entry.expiresAt <= now) {
       THREAD_STARTER_CACHE.delete(cacheKey);
     }
   }
@@ -44,10 +52,11 @@ export async function resolveSlackThreadStarter(params: {
   evictThreadStarterCache();
   const cacheKey = `${params.channelId}:${params.threadTs}`;
   const cached = THREAD_STARTER_CACHE.get(cacheKey);
-  if (cached && Date.now() - cached.cachedAt <= THREAD_STARTER_CACHE_TTL_MS) {
-    return cached.value;
-  }
   if (cached) {
+    const now = asDateTimestampMs(Date.now());
+    if (now !== undefined && cached.expiresAt > now) {
+      return cached.value;
+    }
     THREAD_STARTER_CACHE.delete(cacheKey);
   }
   try {
@@ -78,14 +87,17 @@ export async function resolveSlackThreadStarter(params: {
       ts: message.ts,
       files,
     };
-    if (THREAD_STARTER_CACHE.has(cacheKey)) {
-      THREAD_STARTER_CACHE.delete(cacheKey);
+    const expiresAt = resolveExpiresAtMsFromDurationMs(THREAD_STARTER_CACHE_TTL_MS);
+    if (expiresAt !== undefined) {
+      if (THREAD_STARTER_CACHE.has(cacheKey)) {
+        THREAD_STARTER_CACHE.delete(cacheKey);
+      }
+      THREAD_STARTER_CACHE.set(cacheKey, {
+        value: starter,
+        expiresAt,
+      });
+      evictThreadStarterCache();
     }
-    THREAD_STARTER_CACHE.set(cacheKey, {
-      value: starter,
-      cachedAt: Date.now(),
-    });
-    evictThreadStarterCache();
     return starter;
   } catch (err) {
     logVerbose(