feat(web): older chat timestamps show the date, not just the time

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

@@ -105,7 +105,7 @@ import {
 import { cn } from "~/lib/utils";
 import { useUiStateStore } from "~/uiStateStore";
 import { type TimestampFormat } from "@t3tools/contracts/settings";
-import { formatChatTimestampTooltip, formatShortTimestamp } from "../../timestampFormat";
+import { formatChatTimestampTooltip, formatDayAwareTimestamp } from "../../timestampFormat";
 
 import {
   buildInlineTerminalContextText,
@@ -1038,7 +1038,7 @@ function UserTimelineRow({ row }: { row: Extract<TimelineRow, { kind: "message"
         <div className="flex shrink-0 items-center gap-2">
           <Tooltip>
             <TooltipTrigger render={<p className="text-muted-foreground text-xs tabular-nums" />}>
-              {formatShortTimestamp(row.message.createdAt, ctx.timestampFormat)}
+              {formatDayAwareTimestamp(row.message.createdAt, ctx.timestampFormat)}
             </TooltipTrigger>
             <TooltipPopup>
               {formatChatTimestampTooltip(row.message.createdAt, ctx.timestampFormat)}
@@ -1129,7 +1129,7 @@ function AssistantTimelineRow({ row }: { row: Extract<TimelineRow, { kind: "mess
                 <TooltipTrigger
                   render={<p className="text-muted-foreground text-xs tabular-nums" />}
                 >
-                  {formatShortTimestamp(row.message.updatedAt, ctx.timestampFormat)}
+                  {formatDayAwareTimestamp(row.message.updatedAt, ctx.timestampFormat)}
                 </TooltipTrigger>
                 <TooltipPopup>
                   {formatChatTimestampTooltip(row.message.updatedAt, ctx.timestampFormat)}

apps/web/src/timestampFormat.test.ts

@@ -1,6 +1,7 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vite-plus/test";
 
 import {
+  formatDayAwareTimestamp,
   formatElapsedDurationLabel,
   formatExpiresInLabel,
   formatRelativeTime,
@@ -96,6 +97,55 @@ describe("formatExpiresInLabel", () => {
   });
 });
 
+describe("formatDayAwareTimestamp", () => {
+  // Instants are built with the local-time Date constructor so the
+  // calendar-day boundaries hold in any test timezone or locale.
+  const iso = (y: number, monthIndex: number, d: number, h: number, mi: number) =>
+    new Date(y, monthIndex, d, h, mi).toISOString();
+  const now = new Date(2026, 7, 14, 12, 0).getTime();
+  const time = (isoDate: string) => formatShortTimestamp(isoDate, "12-hour");
+
+  it("shows time only for today", () => {
+    const messageAt = iso(2026, 7, 14, 9, 30);
+    expect(formatDayAwareTimestamp(messageAt, "12-hour", now)).toBe(time(messageAt));
+  });
+
+  it("labels the previous calendar day as yesterday even when under 24h old", () => {
+    const messageAt = iso(2026, 7, 13, 23, 30);
+    const justPastMidnight = new Date(2026, 7, 14, 0, 30).getTime();
+    expect(formatDayAwareTimestamp(messageAt, "12-hour", justPastMidnight)).toBe(
+      `yesterday at ${time(messageAt)}`,
+    );
+  });
+
+  it("prefixes older same-year messages with the numeric date", () => {
+    const messageAt = iso(2026, 7, 12, 12, 34);
+    const datePart = new Intl.DateTimeFormat(undefined, {
+      month: "numeric",
+      day: "numeric",
+    }).format(new Date(messageAt));
+    expect(formatDayAwareTimestamp(messageAt, "12-hour", now)).toBe(
+      `${datePart} ${time(messageAt)}`,
+    );
+  });
+
+  it("includes the year once the calendar year differs", () => {
+    const messageAt = iso(2025, 11, 31, 18, 0);
+    const datePart = new Intl.DateTimeFormat(undefined, {
+      month: "numeric",
+      day: "numeric",
+      year: "numeric",
+    }).format(new Date(messageAt));
+    expect(formatDayAwareTimestamp(messageAt, "12-hour", now)).toBe(
+      `${datePart} ${time(messageAt)}`,
+    );
+  });
+
+  it("returns an empty string for invalid input", () => {
+    expect(formatDayAwareTimestamp("not-a-date", "12-hour", now)).toBe("");
+  });
+});
+
 describe("invalid timestamp inputs", () => {
   it("returns an empty timestamp instead of throwing", () => {
     expect(() => formatTimestamp("not-a-date", "12-hour")).not.toThrow();

apps/web/src/timestampFormat.ts

@@ -91,6 +91,44 @@ export function formatShortTimestamp(isoDate: string, timestampFormat: Timestamp
   return getTimestampFormatter(timestampFormat, false).format(date);
 }
 
+const numericDateFormatter = new Intl.DateTimeFormat(undefined, {
+  month: "numeric",
+  day: "numeric",
+});
+const numericDateWithYearFormatter = new Intl.DateTimeFormat(undefined, {
+  month: "numeric",
+  day: "numeric",
+  year: "numeric",
+});
+
+/**
+ * Chat timestamp that adds the date once the message is no longer from today:
+ * today `12:34 PM`, yesterday `yesterday at 12:34 PM`, older `8/13 12:34 PM`
+ * (locale digit order), with the year included once the calendar year differs.
+ * Boundaries are local calendar days, not 24-hour windows.
+ */
+export function formatDayAwareTimestamp(
+  isoDate: string,
+  timestampFormat: TimestampFormat,
+  nowMs: number = Date.now(),
+): string {
+  const date = parseTimestampDate(isoDate);
+  if (!date) return "";
+  const time = getTimestampFormatter(timestampFormat, false).format(date);
+
+  const now = new Date(nowMs);
+  const startOfToday = new Date(now.getFullYear(), now.getMonth(), now.getDate()).getTime();
+  const startOfMessageDay = new Date(date.getFullYear(), date.getMonth(), date.getDate()).getTime();
+  // Round so DST-shifted 23/25 hour days still count as whole days.
+  const dayDiff = Math.round((startOfToday - startOfMessageDay) / 86_400_000);
+
+  if (dayDiff <= 0) return time;
+  if (dayDiff === 1) return `yesterday at ${time}`;
+  const dateFormatter =
+    date.getFullYear() === now.getFullYear() ? numericDateFormatter : numericDateWithYearFormatter;
+  return `${dateFormatter.format(date)} ${time}`;
+}
+
 /**
  * Format a relative time string from an ISO date.
  * Returns `{ value: "20s", suffix: "ago" }` or `{ value: "just now", suffix: null }`