diff --git a/src/orb/ingest.ts b/src/orb/ingest.ts
index ed89e3483a..c1f5a9e2e4 100644
--- a/src/orb/ingest.ts
+++ b/src/orb/ingest.ts
@@ -10,6 +10,7 @@ const MAX_INSTANCE_ID_CHARS = 64;
 const MAX_HASH_CHARS = 128;
 const MAX_BUCKET_CHARS = 64;
 const MAX_VERDICT_CHARS = 32;
+const MAX_TIMESTAMP_CHARS = 64;
 const VALID_OUTCOMES = new Set(["merged", "closed"]);
 const VALID_REVERSALS = new Set(["none", "reopened", "reverted", "superseded"]);
 // gate_verdict is read downstream as a CLOSED enum by exact equality (analytics.ts foldInstance branches on
@@ -120,6 +121,17 @@ function clampReuseCount(value: unknown): number | null {
   return rounded;
 }
 
+/** Validate a sender-supplied timestamp (#10028). Returns the string unchanged only when it is a
+ *  length-capped, `Date.parse`-able instant; otherwise null. decision_timestamp is the day bucket for the
+ *  public fleet-accuracy trend and the retention rollup key, so a non-instant string would sort above every
+ *  ISO date in the lexicographic window bound, be dropped in JS, and leave a permanent junk `day` in the
+ *  rollup PK. Null lets `COALESCE(decision_timestamp, received_at)` fall back to the server clock, so the
+ *  signal still counts. Mirrors the REUSE_DAY_PATTERN + clampReuseCount pair one field family over. */
+export function normalizeIngestTimestamp(value: unknown): string | null {
+  if (typeof value !== "string" || value.length > MAX_TIMESTAMP_CHARS) return null;
+  return Number.isFinite(Date.parse(value)) ? value : null;
+}
+
 export type OrbIngestResult = { accepted: number } | { error: string };
 
 /** Clamp a sender-supplied cycle time to a plausible range; null for anything implausible/absent. */
@@ -223,9 +235,9 @@ export async function handleOrbIngest(body: string, db: D1Database, presentedIns
           reversal,
           typeof event.gate_reasoncode_bucket === "string" && event.gate_reasoncode_bucket.length <= MAX_BUCKET_CHARS && VALID_REASONCODE_BUCKETS.has(event.gate_reasoncode_bucket) ? event.gate_reasoncode_bucket : null,
           clampCycleMs(event.time_to_close_ms),
-          typeof event.decision_timestamp === "string" ? event.decision_timestamp : null,
-          typeof event.outcome_timestamp === "string" ? event.outcome_timestamp : null,
-          typeof event.outcome_timestamp === "string" ? event.outcome_timestamp : null,
+          normalizeIngestTimestamp(event.decision_timestamp),
+          normalizeIngestTimestamp(event.outcome_timestamp),
+          normalizeIngestTimestamp(event.outcome_timestamp),
         )
         .run();
       if (result.meta.changes > 0) accepted++;
diff --git a/test/integration/orb-ingest.test.ts b/test/integration/orb-ingest.test.ts
index 001bfe7ce6..d99514f5a7 100644
--- a/test/integration/orb-ingest.test.ts
+++ b/test/integration/orb-ingest.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
 import { createApp } from "../../src/api/routes";
-import { handleOrbIngest, MAX_ORB_INGEST_BODY_BYTES, readOrbIngestBody } from "../../src/orb/ingest";
+import { handleOrbIngest, MAX_ORB_INGEST_BODY_BYTES, normalizeIngestTimestamp, readOrbIngestBody } from "../../src/orb/ingest";
 import { createTestEnv, TestD1Database } from "../helpers/d1";
 
 describe("handleOrbIngest()", () => {
@@ -222,6 +222,46 @@ describe("handleOrbIngest()", () => {
     expect(await ingest(db, [ev()])).toBeTruthy();
     expect(call).toBeGreaterThan(0);
   });
+
+  it("#10028: normalizeIngestTimestamp keeps a parseable capped instant and rejects everything else", () => {
+    expect(normalizeIngestTimestamp("2026-07-30T12:00:00.000Z")).toBe("2026-07-30T12:00:00.000Z");
+    expect(normalizeIngestTimestamp("unknown")).toBeNull();
+    expect(normalizeIngestTimestamp("x".repeat(65))).toBeNull();
+    expect(normalizeIngestTimestamp(12345)).toBeNull();
+    expect(normalizeIngestTimestamp(undefined)).toBeNull();
+  });
+
+  it("#10028: a malformed decision_timestamp is stored NULL, the event still accepted", async () => {
+    const db = makeDb();
+    expect(await ingest(db, [ev({ pr_hash: "ts1", decision_timestamp: "unknown" })])).toEqual({ accepted: 1 });
+    expect(await col(db, "ts1", "decision_timestamp")).toBeNull();
+  });
+
+  it("#10028: a well-formed decision_timestamp/outcome_timestamp is stored verbatim in all three columns", async () => {
+    const db = makeDb();
+    const iso = "2026-07-30T12:00:00.000Z";
+    await ingest(db, [ev({ pr_hash: "ts2", decision_timestamp: iso, outcome_timestamp: iso })]);
+    expect(await col(db, "ts2", "decision_timestamp")).toBe(iso);
+    expect(await col(db, "ts2", "outcome_timestamp")).toBe(iso);
+    expect(await col(db, "ts2", "sent_at")).toBe(iso);
+  });
+
+  it("#10028: an over-length (65+ char) timestamp is stored NULL while the event is still accepted", async () => {
+    const db = makeDb();
+    // A parseable ISO prefix but past MAX_TIMESTAMP_CHARS (64): the length cap rejects it before Date.parse.
+    expect(await ingest(db, [ev({ pr_hash: "ts3", decision_timestamp: `2026-07-30T12:00:00.000Z${"0".repeat(50)}` })])).toEqual({ accepted: 1 });
+    expect(await col(db, "ts3", "decision_timestamp")).toBeNull();
+  });
+
+  it("#10028 REGRESSION: a malformed decision_timestamp must not silently drop the signal from the public trend", async () => {
+    const db = makeDb();
+    await ingest(db, [ev({ pr_hash: "ts4", decision_timestamp: "unknown" })]);
+    // COALESCE(decision_timestamp, received_at) must fall back to the server clock and parse to a finite
+    // instant, so substr(...,1,10) is a real day bucket and the signal still counts toward the trend.
+    const coalesced = await col(db, "ts4", "COALESCE(decision_timestamp, received_at)");
+    expect(typeof coalesced).toBe("string");
+    expect(Number.isFinite(Date.parse(String(coalesced)))).toBe(true);
+  });
 });
 
 describe("handleOrbIngest() health ping (#4933)", () => {