diff --git a/extensions/slack/src/sent-thread-cache.test.ts b/extensions/slack/src/sent-thread-cache.test.ts
index f3191ee5f0c..893424788cf 100644
--- a/extensions/slack/src/sent-thread-cache.test.ts
+++ b/extensions/slack/src/sent-thread-cache.test.ts
@@ -108,13 +108,13 @@ describe("slack sent-thread-cache", () => {
       logging: { getChildLogger: () => ({ warn: vi.fn() }) },
     } as never);
 
+    vi.spyOn(Date, "now").mockReturnValue(1_711_406_400_000);
     recordSlackThreadParticipation("A1", "C123", "1700000000.000002");
 
     await vi.waitFor(() => expect(register).toHaveBeenCalledTimes(1));
-    expect(register).toHaveBeenCalledWith(
-      "A1:C123:1700000000.000002",
-      expect.objectContaining({ repliedAt: expect.any(Number) }),
-    );
+    expect(register).toHaveBeenCalledWith("A1:C123:1700000000.000002", {
+      repliedAt: 1_711_406_400_000,
+    });
 
     clearSlackThreadParticipationCache();
     await expect(