diff --git a/src/auto-reply/envelope.test.ts b/src/auto-reply/envelope.test.ts
index 972dc06ff10d..2c2f059f17da 100644
--- a/src/auto-reply/envelope.test.ts
+++ b/src/auto-reply/envelope.test.ts
@@ -79,6 +79,16 @@ describe("formatAgentEnvelope", () => {
     const body = formatAgentEnvelope({ channel: "Telegram", body: "hi" });
     expect(body).toBe("[Telegram] hi");
   });
+
+  it("formats the Unix epoch timestamp", () => {
+    const body = formatAgentEnvelope({
+      channel: "WebChat",
+      timestamp: 0,
+      envelope: { timezone: "utc" },
+      body: "hello",
+    });
+    expect(body).toBe("[WebChat Thu 1970-01-01T00:00:00Z] hello");
+  });
 });
 
 describe("formatInboundEnvelope", () => {
diff --git a/src/auto-reply/envelope.ts b/src/auto-reply/envelope.ts
index 188fa9051b0b..43c2d0623d95 100644
--- a/src/auto-reply/envelope.ts
+++ b/src/auto-reply/envelope.ts
@@ -114,7 +114,7 @@ export function formatEnvelopeTimestamp(
   ts: number | Date | undefined,
   options?: EnvelopeFormatOptions,
 ): string | undefined {
-  if (!ts) {
+  if (ts === undefined) {
     return undefined;
   }
   const resolved = normalizeEnvelopeOptions(options);