feat: Allow Overriding Timestamp Format in Settings

apps/web/src/appSettings.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it } from "vitest";
 
 import {
+  DEFAULT_TIMESTAMP_FORMAT,
   getAppModelOptions,
   normalizeCustomModelSlugs,
   resolveAppModelSelection,
@@ -57,3 +58,9 @@ describe("resolveAppModelSelection", () => {
     expect(resolveAppModelSelection("codex", [], "")).toBe("gpt-5.4");
   });
 });
+
+describe("timestamp format defaults", () => {
+  it("defaults timestamp format to locale", () => {
+    expect(DEFAULT_TIMESTAMP_FORMAT).toBe("locale");
+  });
+});

apps/web/src/appSettings.ts

@@ -7,6 +7,9 @@ import { useLocalStorage } from "./hooks/useLocalStorage";
 const APP_SETTINGS_STORAGE_KEY = "t3code:app-settings:v1";
 const MAX_CUSTOM_MODEL_COUNT = 32;
 export const MAX_CUSTOM_MODEL_LENGTH = 256;
+export const TIMESTAMP_FORMAT_OPTIONS = ["locale", "12-hour", "24-hour"] as const;
+export type TimestampFormat = (typeof TIMESTAMP_FORMAT_OPTIONS)[number];
+export const DEFAULT_TIMESTAMP_FORMAT: TimestampFormat = "locale";
 const BUILT_IN_MODEL_SLUGS_BY_PROVIDER: Record<ProviderKind, ReadonlySet<string>> = {
   codex: new Set(getModelOptions("codex").map((option) => option.slug)),
 };
@@ -22,6 +25,9 @@ const AppSettingsSchema = Schema.Struct({
   enableAssistantStreaming: Schema.Boolean.pipe(
     Schema.withConstructorDefault(() => Option.some(false)),
   ),
+  timestampFormat: Schema.Literals(["locale", "12-hour", "24-hour"]).pipe(
+    Schema.withConstructorDefault(() => Option.some(DEFAULT_TIMESTAMP_FORMAT)),
+  ),
   customCodexModels: Schema.Array(Schema.String).pipe(
     Schema.withConstructorDefault(() => Option.some([])),
   ),

apps/web/src/components/ChatView.tsx

@@ -184,6 +184,7 @@ export default function ChatView({ threadId }: ChatViewProps) {
   const setStoreThreadError = useStore((store) => store.setError);
   const setStoreThreadBranch = useStore((store) => store.setThreadBranch);
   const { settings } = useAppSettings();
+  const timestampFormat = settings.timestampFormat;
   const navigate = useNavigate();
   const rawSearch = useSearch({
     strict: false,
@@ -3244,6 +3245,7 @@ export default function ChatView({ threadId }: ChatViewProps) {
               onImageExpand={onExpandTimelineImage}
               markdownCwd={gitCwd ?? undefined}
               resolvedTheme={resolvedTheme}
+              timestampFormat={timestampFormat}
               workspaceRoot={activeProject?.cwd ?? undefined}
             />
           </div>
@@ -3764,6 +3766,7 @@ export default function ChatView({ threadId }: ChatViewProps) {
             activeProposedPlan={activeProposedPlan}
             markdownCwd={gitCwd ?? undefined}
             workspaceRoot={activeProject?.cwd ?? undefined}
+            timestampFormat={timestampFormat}
             onClose={() => {
               setPlanSidebarOpen(false);
               // Track that the user explicitly dismissed for this turn so auto-open won't fight them.

apps/web/src/components/DiffPanel.tsx

@@ -25,6 +25,8 @@ import { buildPatchCacheKey } from "../lib/diffRendering";
 import { resolveDiffThemeName } from "../lib/diffRendering";
 import { useTurnDiffSummaries } from "../hooks/useTurnDiffSummaries";
 import { useStore } from "../store";
+import { useAppSettings } from "../appSettings";
+import { formatShortTimestamp } from "../timestampFormat";
 import { ToggleGroup, Toggle } from "./ui/toggle-group";
 
 type DiffRenderMode = "stacked" | "split";
@@ -149,13 +151,6 @@ function buildFileDiffRenderKey(fileDiff: FileDiffMetadata): string {
   return fileDiff.cacheKey ?? `${fileDiff.prevName ?? "none"}:${fileDiff.name}`;
 }
 
-function formatTurnChipTimestamp(isoDate: string): string {
-  return new Intl.DateTimeFormat(undefined, {
-    hour: "numeric",
-    minute: "2-digit",
-  }).format(new Date(isoDate));
-}
-
 interface DiffPanelProps {
   mode?: "inline" | "sheet" | "sidebar";
 }
@@ -165,6 +160,7 @@ export { DiffWorkerPoolProvider } from "./DiffWorkerPoolProvider";
 export default function DiffPanel({ mode = "inline" }: DiffPanelProps) {
   const navigate = useNavigate();
   const { resolvedTheme } = useTheme();
+  const { settings } = useAppSettings();
   const [diffRenderMode, setDiffRenderMode] = useState<DiffRenderMode>("stacked");
   const patchViewportRef = useRef<HTMLDivElement>(null);
   const turnStripRef = useRef<HTMLDivElement>(null);
@@ -487,7 +483,7 @@ export default function DiffPanel({ mode = "inline" }: DiffPanelProps) {
                       "?"}
                   </span>
                   <span className="text-[9px] leading-tight opacity-70">
-                    {formatTurnChipTimestamp(summary.completedAt)}
+                    {formatShortTimestamp(summary.completedAt, settings.timestampFormat)}
                   </span>
                 </div>
               </div>

apps/web/src/components/PlanSidebar.tsx

@@ -1,4 +1,5 @@
 import { memo, useState, useCallback, useRef, useEffect } from "react";
+import { type TimestampFormat } from "../appSettings";
 import { Badge } from "./ui/badge";
 import { Button } from "./ui/button";
 import { ScrollArea } from "./ui/scroll-area";
@@ -12,9 +13,9 @@ import {
   PanelRightCloseIcon,
 } from "lucide-react";
 import { cn } from "~/lib/utils";
-import { formatTimestamp } from "../session-logic";
 import type { ActivePlanState } from "../session-logic";
 import type { LatestProposedPlanState } from "../session-logic";
+import { formatTimestamp } from "../timestampFormat";
 import {
   proposedPlanTitle,
   buildProposedPlanMarkdownFilename,
@@ -53,6 +54,7 @@ interface PlanSidebarProps {
   activeProposedPlan: LatestProposedPlanState | null;
   markdownCwd: string | undefined;
   workspaceRoot: string | undefined;
+  timestampFormat: TimestampFormat;
   onClose: () => void;
 }
 
@@ -61,6 +63,7 @@ const PlanSidebar = memo(function PlanSidebar({
   activeProposedPlan,
   markdownCwd,
   workspaceRoot,
+  timestampFormat,
   onClose,
 }: PlanSidebarProps) {
   const [proposedPlanExpanded, setProposedPlanExpanded] = useState(false);
@@ -145,7 +148,7 @@ const PlanSidebar = memo(function PlanSidebar({
           </Badge>
           {activePlan ? (
             <span className="text-[11px] text-muted-foreground/60">
-              {formatTimestamp(activePlan.createdAt)}
+              {formatTimestamp(activePlan.createdAt, timestampFormat)}
             </span>
           ) : null}
         </div>

apps/web/src/components/chat/MessagesTimeline.tsx

@@ -5,7 +5,7 @@ import {
   type VirtualItem,
   useVirtualizer,
 } from "@tanstack/react-virtual";
-import { deriveTimelineEntries, formatElapsed, formatTimestamp } from "../../session-logic";
+import { deriveTimelineEntries, formatElapsed } from "../../session-logic";
 import { AUTO_SCROLL_BOTTOM_THRESHOLD_PX } from "../../chat-scroll";
 import { type TurnDiffSummary } from "../../types";
 import { summarizeTurnDiffStats } from "../../lib/turnDiffTree";
@@ -20,6 +20,8 @@ import { ChangedFilesTree } from "./ChangedFilesTree";
 import { DiffStatLabel, hasNonZeroStat } from "./DiffStatLabel";
 import { MessageCopyButton } from "./MessageCopyButton";
 import { computeMessageDurationStart } from "./MessagesTimeline.logic";
+import { type TimestampFormat } from "../../appSettings";
+import { formatTimestamp } from "../../timestampFormat";
 
 const MAX_VISIBLE_WORK_LOG_ENTRIES = 6;
 const ALWAYS_UNVIRTUALIZED_TAIL_ROWS = 8;
@@ -44,6 +46,7 @@ interface MessagesTimelineProps {
   onImageExpand: (preview: ExpandedImagePreview) => void;
   markdownCwd: string | undefined;
   resolvedTheme: "light" | "dark";
+  timestampFormat: TimestampFormat;
   workspaceRoot: string | undefined;
 }
 
@@ -67,6 +70,7 @@ export const MessagesTimeline = memo(function MessagesTimeline({
   onImageExpand,
   markdownCwd,
   resolvedTheme,
+  timestampFormat,
   workspaceRoot,
 }: MessagesTimelineProps) {
   const timelineRootRef = useRef<HTMLDivElement | null>(null);
@@ -424,7 +428,7 @@ export const MessagesTimeline = memo(function MessagesTimeline({
                     )}
                   </div>
                   <p className="text-right text-[10px] text-muted-foreground/30">
-                    {formatTimestamp(row.message.createdAt)}
+                    {formatTimestamp(row.message.createdAt, timestampFormat)}
                   </p>
                 </div>
               </div>
@@ -515,6 +519,7 @@ export const MessagesTimeline = memo(function MessagesTimeline({
                     row.message.streaming
                       ? formatElapsed(row.durationStart, nowIso)
                       : formatElapsed(row.durationStart, row.message.completedAt),
+                    timestampFormat,
                   )}
                 </p>
               </div>
@@ -650,9 +655,13 @@ function formatWorkingTimer(startIso: string, endIso: string): string | null {
   return seconds > 0 ? `${minutes}m ${seconds}s` : `${minutes}m`;
 }
 
-function formatMessageMeta(createdAt: string, duration: string | null): string {
-  if (!duration) return formatTimestamp(createdAt);
-  return `${formatTimestamp(createdAt)} • ${duration}`;
+function formatMessageMeta(
+  createdAt: string,
+  duration: string | null,
+  timestampFormat: TimestampFormat,
+): string {
+  if (!duration) return formatTimestamp(createdAt, timestampFormat);
+  return `${formatTimestamp(createdAt, timestampFormat)} • ${duration}`;
 }
 
 function workToneClass(tone: "thinking" | "tool" | "info" | "error"): string {

apps/web/src/routes/_chat.settings.tsx

@@ -11,6 +11,13 @@ import { serverConfigQueryOptions } from "../lib/serverReactQuery";
 import { ensureNativeApi } from "../nativeApi";
 import { Button } from "../components/ui/button";
 import { Input } from "../components/ui/input";
+import {
+  Select,
+  SelectItem,
+  SelectPopup,
+  SelectTrigger,
+  SelectValue,
+} from "../components/ui/select";
 import { Switch } from "../components/ui/switch";
 import { APP_VERSION } from "../branding";
 import { SidebarInset } from "~/components/ui/sidebar";
@@ -49,6 +56,12 @@ const MODEL_PROVIDER_SETTINGS: Array<{
   },
 ] as const;
 
+const TIMESTAMP_FORMAT_LABELS = {
+  locale: "System default",
+  "12-hour": "12-hour",
+  "24-hour": "24-hour",
+} as const;
+
 function getCustomModelsForProvider(
   settings: ReturnType<typeof useAppSettings>["settings"],
   provider: ProviderKind,
@@ -210,43 +223,89 @@ function SettingsRouteView() {
               <div className="mb-4">
                 <h2 className="text-sm font-medium text-foreground">Appearance</h2>
                 <p className="mt-1 text-xs text-muted-foreground">
-                  Choose how T3 Code handles light and dark mode.
+                  Choose how T3 Code looks across the app.
                 </p>
               </div>
 
-              <div className="space-y-2" role="radiogroup" aria-label="Theme preference">
-                {THEME_OPTIONS.map((option) => {
-                  const selected = theme === option.value;
-                  return (
-                    <button
-                      key={option.value}
-                      type="button"
-                      role="radio"
-                      aria-checked={selected}
-                      className={`flex w-full items-start justify-between rounded-lg border px-3 py-2 text-left transition-colors ${
-                        selected
-                          ? "border-primary/60 bg-primary/8 text-foreground"
-                          : "border-border bg-background text-muted-foreground hover:bg-accent"
-                      }`}
-                      onClick={() => setTheme(option.value)}
-                    >
-                      <span className="flex flex-col">
-                        <span className="text-sm font-medium">{option.label}</span>
-                        <span className="text-xs">{option.description}</span>
-                      </span>
-                      {selected ? (
-                        <span className="rounded bg-primary/14 px-1.5 py-0.5 text-[10px] font-medium uppercase tracking-wide text-primary">
-                          Selected
+              <div className="space-y-4">
+                <div className="space-y-2" role="radiogroup" aria-label="Theme preference">
+                  {THEME_OPTIONS.map((option) => {
+                    const selected = theme === option.value;
+                    return (
+                      <button
+                        key={option.value}
+                        type="button"
+                        role="radio"
+                        aria-checked={selected}
+                        className={`flex w-full items-start justify-between rounded-lg border px-3 py-2 text-left transition-colors ${
+                          selected
+                            ? "border-primary/60 bg-primary/8 text-foreground"
+                            : "border-border bg-background text-muted-foreground hover:bg-accent"
+                        }`}
+                        onClick={() => setTheme(option.value)}
+                      >
+                        <span className="flex flex-col">
+                          <span className="text-sm font-medium">{option.label}</span>
+                          <span className="text-xs">{option.description}</span>
                         </span>
-                      ) : null}
-                    </button>
-                  );
-                })}
-              </div>
+                        {selected ? (
+                          <span className="rounded bg-primary/14 px-1.5 py-0.5 text-[10px] font-medium uppercase tracking-wide text-primary">
+                            Selected
+                          </span>
+                        ) : null}
+                      </button>
+                    );
+                  })}
+                </div>
 
-              <p className="mt-4 text-xs text-muted-foreground">
-                Active theme: <span className="font-medium text-foreground">{resolvedTheme}</span>
-              </p>
+                <p className="text-xs text-muted-foreground">
+                  Active theme: <span className="font-medium text-foreground">{resolvedTheme}</span>
+                </p>
+
+                <div className="flex items-center justify-between rounded-lg border border-border bg-background px-3 py-2">
+                  <div>
+                    <p className="text-sm font-medium text-foreground">Timestamp format</p>
+                    <p className="text-xs text-muted-foreground">
+                      System default follows your browser or OS time format. <code>12-hour</code>{" "}
+                      and <code>24-hour</code> force the hour cycle.
+                    </p>
+                  </div>
+                  <Select
+                    value={settings.timestampFormat}
+                    onValueChange={(value) => {
+                      if (value !== "locale" && value !== "12-hour" && value !== "24-hour") return;
+                      updateSettings({
+                        timestampFormat: value,
+                      });
+                    }}
+                  >
+                    <SelectTrigger className="w-40" aria-label="Timestamp format">
+                      <SelectValue>{TIMESTAMP_FORMAT_LABELS[settings.timestampFormat]}</SelectValue>
+                    </SelectTrigger>
+                    <SelectPopup align="end">
+                      <SelectItem value="locale">{TIMESTAMP_FORMAT_LABELS.locale}</SelectItem>
+                      <SelectItem value="12-hour">{TIMESTAMP_FORMAT_LABELS["12-hour"]}</SelectItem>
+                      <SelectItem value="24-hour">{TIMESTAMP_FORMAT_LABELS["24-hour"]}</SelectItem>
+                    </SelectPopup>
+                  </Select>
+                </div>
+
+                {settings.timestampFormat !== defaults.timestampFormat ? (
+                  <div className="flex justify-end">
+                    <Button
+                      size="xs"
+                      variant="outline"
+                      onClick={() =>
+                        updateSettings({
+                          timestampFormat: defaults.timestampFormat,
+                        })
+                      }
+                    >
+                      Restore default
+                    </Button>
+                  </div>
+                ) : null}
+              </div>
             </section>
 
             <section className="rounded-2xl border border-border bg-card p-5">

apps/web/src/session-logic.ts

@@ -89,14 +89,6 @@ export type TimelineEntry =
       entry: WorkLogEntry;
     };
 
-export function formatTimestamp(isoDate: string): string {
-  return new Intl.DateTimeFormat(undefined, {
-    hour: "numeric",
-    minute: "2-digit",
-    second: "2-digit",
-  }).format(new Date(isoDate));
-}
-
 export function formatDuration(durationMs: number): string {
   if (!Number.isFinite(durationMs) || durationMs < 0) return "0ms";
   if (durationMs < 1_000) return `${Math.max(1, Math.round(durationMs))}ms`;