diff --git a/packages/orm/src/client/zod/factory.ts b/packages/orm/src/client/zod/factory.ts
index 225bf6e34..066541f67 100644
--- a/packages/orm/src/client/zod/factory.ts
+++ b/packages/orm/src/client/zod/factory.ts
@@ -84,6 +84,37 @@ export function createQuerySchemaFactory(clientOrSchema: any, options?: any) {
     return new ZodSchemaFactory(clientOrSchema, options);
 }
 
+/**
+ * Builds a `DateTime` value schema that accepts a `Date` object or any string
+ * the JS `Date` constructor parses, and coerces it to a `Date`. ISO datetime,
+ * ISO date, and time-only strings (e.g. `"09:00:00"` for `@db.Time` fields,
+ * anchored to the Unix epoch) are the documented happy paths; other formats
+ * accepted by `new Date(...)` also pass through, mirroring Prisma's pre-3.5
+ * behaviour. Strings the engine can't parse fall through and are rejected by
+ * `z.date()` with the standard error.
+ *
+ * @see https://github.com/zenstackhq/zenstack/issues/2631
+ */
+export function coercedDateTimeSchema(): ZodType {
+    // The schema keeps the original `z.iso.datetime() | z.iso.date() | z.date()`
+    // union so the generated OpenAPI spec still documents the accepted ISO
+    // forms. Preprocess runs first and coerces strings into `Date` objects,
+    // so the union's `z.date()` arm catches everything that successfully
+    // parses — including non-ISO formats like `"2024/01/15"` for Prisma
+    // compatibility (rejected with the standard error if `new Date(...)`
+    // returns Invalid Date).
+    return z.preprocess((val) => {
+        if (typeof val !== 'string') return val;
+        if (/^\d{2}:\d{2}(?::\d{2}(?:\.\d+)?)?(?:Z|[+-]\d\d(?::\d\d)?)?$/.test(val)) {
+            const hasTz = val.endsWith('Z') || /[+-]\d\d(?::\d\d)?$/.test(val);
+            const d = new Date(`1970-01-01T${val}${hasTz ? '' : 'Z'}`);
+            return isNaN(d.getTime()) ? val : d;
+        }
+        const d = new Date(val);
+        return isNaN(d.getTime()) ? val : d;
+    }, z.union([z.iso.datetime(), z.iso.date(), z.date()]));
+}
+
 /**
  * Options for creating Zod schemas.
  */
@@ -854,7 +885,7 @@ export class ZodSchemaFactory<
 
     @cache()
     private makeDateTimeValueSchema(): ZodType {
-        const schema = z.union([z.iso.datetime(), z.iso.date(), z.date()]);
+        const schema = coercedDateTimeSchema();
         this.registerSchema('DateTime', schema);
         return schema;
     }
diff --git a/tests/regression/test/issue-2631.test.ts b/tests/regression/test/issue-2631.test.ts
new file mode 100644
index 000000000..84d270be6
--- /dev/null
+++ b/tests/regression/test/issue-2631.test.ts
@@ -0,0 +1,50 @@
+import { createTestClient } from '@zenstackhq/testtools';
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+
+// Regression for #2631: ZenStack 3.5+ replaced Prisma's permissive
+// datetime input coercion with a strict zod union, breaking every caller
+// that passed ISO strings to `DateTime` fields. `DateTime` inputs now
+// coerce strings the JS `Date` constructor parses back to `Date`,
+// mirroring Prisma's pre-3.5 behaviour.
+describe('Issue 2631 — DateTime input coercion', () => {
+    const schema = `
+model Event {
+    id      Int      @id @default(autoincrement())
+    label   String
+    when    DateTime
+}
+    `;
+
+    let db: any;
+
+    beforeEach(async () => {
+        db = await createTestClient(schema, { usePrismaPush: true, provider: 'sqlite' });
+    });
+    afterEach(async () => db?.$disconnect());
+
+    it('accepts a Date object', async () => {
+        const e = await db.event.create({ data: { label: 'date', when: new Date('2024-01-15T10:30:00Z') } });
+        expect(e.when).toBeInstanceOf(Date);
+    });
+
+    it('accepts an ISO datetime string and coerces to Date', async () => {
+        const e = await db.event.create({ data: { label: 'iso', when: '2024-01-15T10:30:00.000Z' } });
+        expect(e.when).toBeInstanceOf(Date);
+    });
+
+    it('accepts an ISO date string and coerces to Date', async () => {
+        const e = await db.event.create({ data: { label: 'date-only', when: '2024-01-15' } });
+        expect(e.when).toBeInstanceOf(Date);
+    });
+
+    it('accepts a bare time-only string anchored to the Unix epoch', async () => {
+        const e = await db.event.create({ data: { label: 'time-only', when: '09:30:00' } });
+        expect(e.when).toBeInstanceOf(Date);
+        expect((e.when as Date).getUTCHours()).toBe(9);
+        expect((e.when as Date).getUTCMinutes()).toBe(30);
+    });
+
+    it('rejects a non-parseable string', async () => {
+        await expect(db.event.create({ data: { label: 'junk', when: 'not-a-date' as any } })).rejects.toThrow();
+    });
+});