diff --git a/apps/web/src/components/settings/DiagnosticsSettings.tsx b/apps/web/src/components/settings/DiagnosticsSettings.tsx
index 92d8d3f6827..b1a54feb718 100644
--- a/apps/web/src/components/settings/DiagnosticsSettings.tsx
+++ b/apps/web/src/components/settings/DiagnosticsSettings.tsx
@@ -23,7 +23,7 @@ import * as Option from "effect/Option";
 
 import { cn } from "../../lib/utils";
 import { resolveAndPersistPreferredEditor } from "../../editorPreferences";
-import { formatRelativeTime } from "../../timestampFormat";
+import { formatRelativeTimeLabel, getRelativeTimeState } from "../../timestampFormat";
 import { useEnvironmentQuery } from "../../state/query";
 import {
   primaryServerAvailableEditorsAtom,
@@ -65,8 +65,7 @@ function formatBytes(value: number): string {
 
 function formatRelative(value: DateTime.Utc | null): string {
   if (!value) return "No trace records";
-  const relative = formatRelativeTime(DateTime.formatIso(value));
-  return relative.suffix ? `${relative.value} ${relative.suffix}` : relative.value;
+  return formatRelativeTimeLabel(DateTime.formatIso(value));
 }
 
 function formatRelativeNoWrap(value: DateTime.Utc | null): string {
@@ -755,12 +754,16 @@ function ProcessResourceHistoryTable({
 
 function DiagnosticsLastChecked({ checkedAt }: { checkedAt: DateTime.Utc | null }) {
   useRelativeTimeTick();
-  const relative = checkedAt ? formatRelativeTime(DateTime.formatIso(checkedAt)) : null;
+  const relative = getRelativeTimeState(checkedAt ? DateTime.formatIso(checkedAt) : null);
 
-  if (!relative) {
+  if (relative.status === "missing") {
     return <span className="text-[11px] text-muted-foreground/50">Checking</span>;
   }
 
+  if (relative.status === "invalid") {
+    return <span className="text-[11px] text-muted-foreground/50">Checked unavailable</span>;
+  }
+
   return (
     <span className="text-[11px] text-muted-foreground/60">
       {relative.suffix ? (
diff --git a/apps/web/src/components/settings/SettingsPanels.tsx b/apps/web/src/components/settings/SettingsPanels.tsx
index 40017d56314..4ac3ce359e0 100644
--- a/apps/web/src/components/settings/SettingsPanels.tsx
+++ b/apps/web/src/components/settings/SettingsPanels.tsx
@@ -58,7 +58,7 @@ import {
 import { usePrimaryEnvironment } from "../../state/environments";
 import { useProjects } from "../../state/entities";
 import { useArchivedThreadSnapshots } from "../../lib/archivedThreadsState";
-import { formatRelativeTime, formatRelativeTimeLabel } from "../../timestampFormat";
+import { formatRelativeTimeLabel, getRelativeTimeState } from "../../timestampFormat";
 import { Button } from "../ui/button";
 import { DraftInput } from "../ui/draft-input";
 import { Select, SelectItem, SelectPopup, SelectTrigger, SelectValue } from "../ui/select";
@@ -134,12 +134,16 @@ const PROVIDER_SETTINGS = DRIVER_OPTIONS.map((definition) => ({
 
 function ProviderLastChecked({ lastCheckedAt }: { lastCheckedAt: string | null }) {
   useRelativeTimeTick();
-  const lastCheckedRelative = lastCheckedAt ? formatRelativeTime(lastCheckedAt) : null;
+  const lastCheckedRelative = getRelativeTimeState(lastCheckedAt);
 
-  if (!lastCheckedRelative) {
+  if (lastCheckedRelative.status === "missing") {
     return null;
   }
 
+  if (lastCheckedRelative.status === "invalid") {
+    return <span className="text-[11px] text-muted-foreground/50">Checked unavailable</span>;
+  }
+
   return (
     <span className="text-[11px] text-muted-foreground/60">
       {lastCheckedRelative.suffix ? (
diff --git a/apps/web/src/timestampFormat.test.ts b/apps/web/src/timestampFormat.test.ts
index c68d5b7ee46..c2fe4b62714 100644
--- a/apps/web/src/timestampFormat.test.ts
+++ b/apps/web/src/timestampFormat.test.ts
@@ -3,7 +3,13 @@ import { afterEach, beforeEach, describe, expect, it, vi } from "vite-plus/test"
 import {
   formatElapsedDurationLabel,
   formatExpiresInLabel,
+  formatRelativeTime,
+  formatRelativeTimeLabel,
+  formatRelativeTimeUntil,
   formatRelativeTimeUntilLabel,
+  formatShortTimestamp,
+  formatTimestamp,
+  getRelativeTimeState,
   getTimestampFormatOptions,
 } from "./timestampFormat";
 
@@ -90,6 +96,60 @@ describe("formatExpiresInLabel", () => {
   });
 });
 
+describe("invalid timestamp inputs", () => {
+  it("returns an empty timestamp instead of throwing", () => {
+    expect(() => formatTimestamp("not-a-date", "12-hour")).not.toThrow();
+    expect(formatTimestamp("not-a-date", "12-hour")).toBe("");
+  });
+
+  it("returns an empty short timestamp instead of throwing", () => {
+    expect(() => formatShortTimestamp("not-a-date", "12-hour")).not.toThrow();
+    expect(formatShortTimestamp("not-a-date", "12-hour")).toBe("");
+  });
+
+  it("returns an empty relative time label instead of a NaN label", () => {
+    expect(formatRelativeTime("not-a-date")).toBeNull();
+    expect(formatRelativeTimeLabel("not-a-date")).toBe("");
+  });
+
+  it("distinguishes missing and invalid relative time state", () => {
+    expect(getRelativeTimeState(null)).toEqual({ status: "missing" });
+    expect(getRelativeTimeState("not-a-date")).toEqual({ status: "invalid" });
+  });
+
+  it("returns an empty elapsed duration instead of a NaN label", () => {
+    expect(formatElapsedDurationLabel("not-a-date")).toBe("");
+  });
+
+  it("returns an empty relative time until label instead of a NaN label", () => {
+    expect(formatRelativeTimeUntil("not-a-date")).toBeNull();
+    expect(formatRelativeTimeUntilLabel("not-a-date")).toBe("");
+  });
+
+  it("returns an empty expires-in label instead of a NaN label", () => {
+    expect(formatExpiresInLabel("not-a-date")).toBe("");
+  });
+});
+
+describe("getRelativeTimeState", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-04-07T12:00:00.000Z"));
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("returns relative parts for valid timestamps", () => {
+    expect(getRelativeTimeState("2026-04-07T11:45:00.000Z")).toEqual({
+      status: "relative",
+      value: "15m",
+      suffix: "ago",
+    });
+  });
+});
+
 describe("formatElapsedDurationLabel", () => {
   beforeEach(() => {
     vi.useFakeTimers();
diff --git a/apps/web/src/timestampFormat.ts b/apps/web/src/timestampFormat.ts
index 28facf877e5..31cc41798eb 100644
--- a/apps/web/src/timestampFormat.ts
+++ b/apps/web/src/timestampFormat.ts
@@ -40,8 +40,15 @@ function getTimestampFormatter(
   return formatter;
 }
 
+function parseTimestampDate(isoDate: string): Date | null {
+  const date = new Date(isoDate);
+  return Number.isNaN(date.getTime()) ? null : date;
+}
+
 export function formatTimestamp(isoDate: string, timestampFormat: TimestampFormat): string {
-  return getTimestampFormatter(timestampFormat, true).format(new Date(isoDate));
+  const date = parseTimestampDate(isoDate);
+  if (!date) return "";
+  return getTimestampFormatter(timestampFormat, true).format(date);
 }
 
 const monthNameFormatter = new Intl.DateTimeFormat(undefined, { month: "long" });
@@ -69,8 +76,8 @@ export function formatChatTimestampTooltip(
   isoDate: string,
   timestampFormat: TimestampFormat,
 ): string {
-  const date = new Date(isoDate);
-  if (Number.isNaN(date.getTime())) return "";
+  const date = parseTimestampDate(isoDate);
+  if (!date) return "";
   const time = formatShortTimestamp(isoDate, timestampFormat);
   const day = date.getDate();
   const month = monthNameFormatter.format(date);
@@ -79,7 +86,9 @@ export function formatChatTimestampTooltip(
 }
 
 export function formatShortTimestamp(isoDate: string, timestampFormat: TimestampFormat): string {
-  return getTimestampFormatter(timestampFormat, false).format(new Date(isoDate));
+  const date = parseTimestampDate(isoDate);
+  if (!date) return "";
+  return getTimestampFormatter(timestampFormat, false).format(date);
 }
 
 /**
@@ -87,8 +96,16 @@ export function formatShortTimestamp(isoDate: string, timestampFormat: Timestamp
  * Returns `{ value: "20s", suffix: "ago" }` or `{ value: "just now", suffix: null }`
  * so callers can style the numeric portion independently.
  */
-export function formatRelativeTime(isoDate: string): { value: string; suffix: string | null } {
-  const diffMs = Date.now() - new Date(isoDate).getTime();
+type RelativeTimeParts = { value: string; suffix: string | null };
+export type RelativeTimeState =
+  | { status: "missing" }
+  | { status: "invalid" }
+  | { status: "relative"; value: string; suffix: string | null };
+
+export function formatRelativeTime(isoDate: string): RelativeTimeParts | null {
+  const date = parseTimestampDate(isoDate);
+  if (!date) return null;
+  const diffMs = Date.now() - date.getTime();
   if (diffMs < 0) return { value: "just now", suffix: null };
   const seconds = Math.floor(diffMs / 1000);
   if (seconds < 60) return { value: "just now", suffix: null };
@@ -102,15 +119,25 @@ export function formatRelativeTime(isoDate: string): { value: string; suffix: st
 
 export function formatRelativeTimeLabel(isoDate: string) {
   const relative = formatRelativeTime(isoDate);
+  if (!relative) return "";
   return relative.suffix ? `${relative.value} ${relative.suffix}` : relative.value;
 }
 
+export function getRelativeTimeState(isoDate: string | null): RelativeTimeState {
+  if (!isoDate) return { status: "missing" };
+  const relative = formatRelativeTime(isoDate);
+  if (!relative) return { status: "invalid" };
+  return { status: "relative", ...relative };
+}
+
 /**
  * Relative elapsed duration since an ISO instant, without an "ago" suffix.
  * Useful for labels like "Connected for 3m".
  */
 export function formatElapsedDurationLabel(isoDate: string, nowMs: number = Date.now()): string {
-  const diffMs = nowMs - new Date(isoDate).getTime();
+  const date = parseTimestampDate(isoDate);
+  if (!date) return "";
+  const diffMs = nowMs - date.getTime();
   if (diffMs <= 0) return "just now";
 
   const seconds = Math.floor(diffMs / 1000);
@@ -130,8 +157,10 @@ export function formatElapsedDurationLabel(isoDate: string, nowMs: number = Date
 /**
  * Relative time until an ISO instant (e.g. expiry). Mirrors {@link formatRelativeTime} but for future times.
  */
-export function formatRelativeTimeUntil(isoDate: string): { value: string; suffix: string | null } {
-  const diffMs = new Date(isoDate).getTime() - Date.now();
+export function formatRelativeTimeUntil(isoDate: string): RelativeTimeParts | null {
+  const date = parseTimestampDate(isoDate);
+  if (!date) return null;
+  const diffMs = date.getTime() - Date.now();
   if (diffMs <= 0) return { value: "Expired", suffix: null };
   const seconds = Math.floor(diffMs / 1000);
   if (seconds < 5) return { value: "Soon", suffix: null };
@@ -146,6 +175,7 @@ export function formatRelativeTimeUntil(isoDate: string): { value: string; suffi
 
 export function formatRelativeTimeUntilLabel(isoDate: string): string {
   const relative = formatRelativeTimeUntil(isoDate);
+  if (!relative) return "";
   return relative.suffix ? `${relative.value} ${relative.suffix}` : relative.value;
 }
 
@@ -154,7 +184,9 @@ export function formatRelativeTimeUntilLabel(isoDate: string): string {
  * Pass `nowMs` when a parent tick drives re-renders so the diff matches that snapshot.
  */
 export function formatExpiresInLabel(isoDate: string, nowMs: number = Date.now()): string {
-  const diffMs = new Date(isoDate).getTime() - nowMs;
+  const date = parseTimestampDate(isoDate);
+  if (!date) return "";
+  const diffMs = date.getTime() - nowMs;
   if (diffMs <= 0) return "Expired";
 
   const totalSeconds = Math.floor(diffMs / 1000);