diff --git a/src/lib/pipeline/schemas.ts b/src/lib/pipeline/schemas.ts
index 942de78..4ce63ab 100644
--- a/src/lib/pipeline/schemas.ts
+++ b/src/lib/pipeline/schemas.ts
@@ -18,9 +18,21 @@ export function optionalText(max: number) {
     .transform((value) => (value ? value : value === undefined ? undefined : null));
 }
 
+/** True when YYYY-MM-DD names a day that exists. Date would roll 2026-02-31 over into March. */
+function isCalendarDay(value: string) {
+  const [year, month, day] = value.split("-").map(Number) as [number, number, number];
+  const date = new Date(Date.UTC(year, month - 1, day));
+  return date.getUTCFullYear() === year && date.getUTCMonth() === month - 1 && date.getUTCDate() === day;
+}
+
 /** Accepts an ISO timestamp or an HTML date-input value (YYYY-MM-DD, interpreted as UTC midnight). */
 export const optionalDate = z
-  .union([z.string().datetime({ offset: true }), z.string().regex(/^\d{4}-\d{2}-\d{2}$/, "Use YYYY-MM-DD"), z.literal(""), z.null()])
+  .union([
+    z.string().datetime({ offset: true }),
+    z.string().regex(/^\d{4}-\d{2}-\d{2}$/, "Use YYYY-MM-DD").refine(isCalendarDay, "Enter a date that exists."),
+    z.literal(""),
+    z.null(),
+  ])
   .optional()
   .transform((value) => {
     if (value === undefined) return undefined;
diff --git a/tests/opportunity-pipeline.test.ts b/tests/opportunity-pipeline.test.ts
index 2cc9c90..d7a1d86 100644
--- a/tests/opportunity-pipeline.test.ts
+++ b/tests/opportunity-pipeline.test.ts
@@ -131,6 +131,15 @@ describe("opportunity input validation", () => {
     expect(parsed.nextActionAt).toEqual(new Date("2026-10-01T00:00:00.000Z"));
   });
 
+  it("rejects date-only values for days that do not exist", () => {
+    for (const nextActionAt of ["2026-02-31", "2026-04-31", "2026-13-01", "2026-00-10"]) {
+      expect(createOpportunitySchema.safeParse({ company: { name: "Acme" }, nextActionAt }).success).toBe(false);
+    }
+    expect(createOpportunitySchema.parse({ company: { name: "Acme" }, nextActionAt: "2028-02-29" }).nextActionAt).toEqual(
+      new Date("2028-02-29T00:00:00.000Z"),
+    );
+  });
+
   it("requires a version and at least one field for updates, and a known stage for stage changes", () => {
     expect(updateOpportunitySchema.safeParse({ expectedVersion: 1 }).success).toBe(false);
     expect(updateOpportunitySchema.safeParse({ priority: "HIGH" }).success).toBe(false);