diff --git a/packages/signals/README.md b/packages/signals/README.md
index 6219ff2..7739f5a 100644
--- a/packages/signals/README.md
+++ b/packages/signals/README.md
@@ -74,7 +74,7 @@ The person an event belongs to always comes from the token: Didit overwrites any
 | `validation` | The event failed validation; `error.errors` has Didit's per-field details |
 | `dropped` | The queue was full and the event was dropped before it was sent |
 
-`submitEvent` itself throws a `DiditEventError` only for a missing token (`invalid_token`), a missing `category` or `actionType` (`validation`), or an environment without `fetch` (`network`).
+`submitEvent` itself throws a `DiditEventError` only for a missing token (`invalid_token`), a missing `category` or `actionType` or an unusable `occurredAt` Date (`validation`), or an environment without `fetch` (`network`).
 
 ## API
 
diff --git a/packages/signals/src/__tests__/events.test.ts b/packages/signals/src/__tests__/events.test.ts
index 55696fe..c1d3665 100644
--- a/packages/signals/src/__tests__/events.test.ts
+++ b/packages/signals/src/__tests__/events.test.ts
@@ -516,6 +516,12 @@ describe("submitEvent input", () => {
     );
   });
 
+  it("refuses an unusable occurredAt with a typed validation error", () => {
+    expect(() =>
+      submitEvent({ monitoringToken: "valid", event: { category: "auth", actionType: "login", occurredAt: new Date("not a date") } })
+    ).toThrow(expect.objectContaining({ name: "DiditEventError", type: "validation" }));
+  });
+
   it("returns the integrator's event id, or a new one per event", () => {
     const event: DiditEvent = { category: "auth", actionType: "login" };
 
diff --git a/packages/signals/src/events.ts b/packages/signals/src/events.ts
index 82d55f3..18bdf15 100644
--- a/packages/signals/src/events.ts
+++ b/packages/signals/src/events.ts
@@ -434,6 +434,9 @@ export function submitEvent(options: SubmitEventOptions): string {
   if (!isRecord(event) || !event.category || typeof event.actionType !== "string" || !event.actionType.trim()) {
     throw new DiditEventError("validation", "event.category and event.actionType are required.");
   }
+  if (event.occurredAt instanceof Date && Number.isNaN(event.occurredAt.getTime())) {
+    throw new DiditEventError("validation", "event.occurredAt must be a valid date.");
+  }
   if (typeof fetch !== "function") {
     throw new DiditEventError("network", "fetch is not available in this environment.");
   }
diff --git a/packages/signals/src/types.ts b/packages/signals/src/types.ts
index 3144d09..3aedccb 100644
--- a/packages/signals/src/types.ts
+++ b/packages/signals/src/types.ts
@@ -71,7 +71,7 @@ export interface DiditEvent {
    * same content is a harmless replay; with different content it is rejected.
    */
   eventId?: string;
-  /** When it happened. @default the time submitEvent was called */
+  /** When it happened. An invalid Date is refused with a "validation" error. @default the time submitEvent was called */
   occurredAt?: Date | string;
   /** IANA time zone of the end user. @default the browser's time zone */
   timeZone?: string;