diff --git a/packages/core-shared/src/lib/date.test.ts b/packages/core-shared/src/lib/date.test.ts
new file mode 100644
index 0000000..bfcc316
--- /dev/null
+++ b/packages/core-shared/src/lib/date.test.ts
@@ -0,0 +1,23 @@
+import { describe, expect, it } from "vitest";
+import { toIsoString } from "./date";
+
+describe("toIsoString", () => {
+  it("converts a Date to ISO string", () => {
+    const d = new Date("2026-05-04T12:00:00.000Z");
+    expect(toIsoString(d)).toBe("2026-05-04T12:00:00.000Z");
+  });
+
+  it("passes through an existing ISO string", () => {
+    expect(toIsoString("2026-05-04T12:00:00.000Z")).toBe(
+      "2026-05-04T12:00:00.000Z",
+    );
+  });
+
+  it("returns null for null input", () => {
+    expect(toIsoString(null)).toBeNull();
+  });
+
+  it("returns null for undefined input", () => {
+    expect(toIsoString(undefined)).toBeNull();
+  });
+});
diff --git a/packages/core-shared/src/lib/date.ts b/packages/core-shared/src/lib/date.ts
new file mode 100644
index 0000000..6e3ee48
--- /dev/null
+++ b/packages/core-shared/src/lib/date.ts
@@ -0,0 +1,5 @@
+export function toIsoString(input: Date | string | null | undefined): string | null {
+  if (input === null || input === undefined) return null;
+  if (input instanceof Date) return input.toISOString();
+  return input;
+}