diff --git a/src/components/core/MonoText.tsx b/src/components/core/MonoText.tsx
index 3d878f6e..ca10ebf1 100644
--- a/src/components/core/MonoText.tsx
+++ b/src/components/core/MonoText.tsx
@@ -1,10 +1,13 @@
 import { Text } from '@radix-ui/themes';
-import type { ComponentProps } from 'react';
+import { forwardRef, type ComponentProps } from 'react';
 
 type MonoTextProps = ComponentProps<typeof Text>;
 
-export function MonoText(props: MonoTextProps) {
-  return (
-    <Text {...props} style={{ fontFamily: 'var(--code-font-family)', ...props.style }} />
-  );
-} 
\ No newline at end of file
+// forwardRef so Radix primitives (Tooltip, etc.) can anchor to it.
+export const MonoText = forwardRef<HTMLSpanElement, MonoTextProps>(
+  function MonoText(props, ref) {
+    return (
+      <Text ref={ref} {...props} style={{ fontFamily: 'var(--code-font-family)', ...props.style }} />
+    );
+  }
+);
diff --git a/src/components/features/products/object-browser/DirectoryRow.test.tsx b/src/components/features/products/object-browser/DirectoryRow.test.tsx
new file mode 100644
index 00000000..26baf03e
--- /dev/null
+++ b/src/components/features/products/object-browser/DirectoryRow.test.tsx
@@ -0,0 +1,63 @@
+import { render, screen } from "@testing-library/react";
+import { Theme } from "@radix-ui/themes";
+import { DirectoryRow } from "./DirectoryRow";
+import type { Product } from "@/types";
+
+jest.mock("next/navigation", () => ({
+  useRouter: () => ({ refresh: jest.fn() }),
+}));
+jest.mock("@/components/features/uploader", () => ({
+  useUploadManager: () => ({
+    cancelUpload: jest.fn(),
+    retryUpload: jest.fn(),
+    getUploadsForScope: () => [],
+    deleteObject: jest.fn(),
+    deletePrefix: jest.fn(),
+  }),
+  useS3Credentials: () => ({ getCredentials: () => null }),
+}));
+
+const product = {
+  account_id: "cholmes",
+  product_id: "overture",
+} as Product;
+
+const renderRow = (item: Parameters<typeof DirectoryRow>[0]["item"]) =>
+  render(
+    <Theme>
+      <DirectoryRow
+        item={item}
+        index={0}
+        itemsLength={1}
+        itemHeight={40}
+        product={product}
+      />
+    </Theme>
+  );
+
+describe("DirectoryRow last modified", () => {
+  it("shows a relative time with the UTC timestamp as its title", () => {
+    renderRow({
+      name: "catalog.json",
+      path: "catalog.json",
+      size: 1234,
+      updated_at: new Date(Date.now() - 21 * 24 * 3600 * 1000).toISOString(),
+      isDirectory: false,
+    });
+    expect(screen.getByText("3 weeks ago")).toHaveAttribute(
+      "title",
+      expect.stringContaining("UTC")
+    );
+  });
+
+  it("omits it for directories, whose mtime is synthetic", () => {
+    renderRow({
+      name: "tiles",
+      path: "tiles/",
+      size: 0,
+      updated_at: new Date().toISOString(),
+      isDirectory: true,
+    });
+    expect(screen.queryByText(/ago$/)).toBeNull();
+  });
+});
diff --git a/src/components/features/products/object-browser/DirectoryRow.tsx b/src/components/features/products/object-browser/DirectoryRow.tsx
index 7b5cb144..57eddf3b 100644
--- a/src/components/features/products/object-browser/DirectoryRow.tsx
+++ b/src/components/features/products/object-browser/DirectoryRow.tsx
@@ -24,7 +24,7 @@ import { useRouter } from "next/navigation";
 import { MonoText } from "@/components/core";
 import type { FileNode } from "./utils";
 import type { Product } from "@/types";
-import { formatBytes } from "@/lib/format";
+import { formatBytes, formatDate, formatRelativeTime } from "@/lib/format";
 import { objectUrl } from "@/lib/urls";
 import styles from "./ObjectBrowser.module.css";
 import { useState } from "react";
@@ -213,6 +213,23 @@ export function DirectoryRow({
                 </MonoText>
               )}
 
+              {/* Last modified — directories have no real mtime, so files only.
+                  suppressHydrationWarning: "now" differs between server and
+                  client render, so a just-uploaded file can disagree by seconds. */}
+              {!item.isDirectory && item.updated_at && (
+                <Tooltip content={formatDate(item.updated_at, true)}>
+                  <MonoText
+                    color="gray"
+                    size="1"
+                    title={formatDate(item.updated_at, true)}
+                    suppressHydrationWarning
+                    style={{ flexShrink: 0, whiteSpace: "nowrap" }}
+                  >
+                    {formatRelativeTime(item.updated_at)}
+                  </MonoText>
+                </Tooltip>
+              )}
+
               {/* Upload control buttons */}
               {!item.isDirectory && isUploading && uploadItem && (
                 <Flex gap="1">
diff --git a/src/lib/format.test.ts b/src/lib/format.test.ts
new file mode 100644
index 00000000..abb2ce2a
--- /dev/null
+++ b/src/lib/format.test.ts
@@ -0,0 +1,21 @@
+import { formatRelativeTime } from "./format";
+
+describe("formatRelativeTime", () => {
+  const now = new Date("2024-06-15T12:00:00Z");
+  const ago = (ms: number) => new Date(now.getTime() - ms).toISOString();
+
+  it.each([
+    [ago(30_000), "30 seconds ago"],
+    [ago(5 * 60_000), "5 minutes ago"],
+    [ago(3 * 3600_000), "3 hours ago"],
+    [ago(2 * 24 * 3600_000), "2 days ago"],
+    [ago(21 * 24 * 3600_000), "3 weeks ago"],
+    [ago(400 * 24 * 3600_000), "last year"],
+  ])("%s -> %s", (date, expected) => {
+    expect(formatRelativeTime(date, now)).toBe(expected);
+  });
+
+  it("returns empty string for an unparseable date", () => {
+    expect(formatRelativeTime("not a date", now)).toBe("");
+  });
+});
diff --git a/src/lib/format.ts b/src/lib/format.ts
index 092bb324..69a6cc4c 100644
--- a/src/lib/format.ts
+++ b/src/lib/format.ts
@@ -25,6 +25,34 @@ export function formatDateSSR(date: string): string {
   return `${day} ${month} ${year}`;
 }
 
+const RELATIVE_UNITS: [Intl.RelativeTimeFormatUnit, number][] = [
+  ['year', 365 * 24 * 3600],
+  ['month', 30 * 24 * 3600],
+  ['week', 7 * 24 * 3600],
+  ['day', 24 * 3600],
+  ['hour', 3600],
+  ['minute', 60],
+  ['second', 1],
+];
+
+/**
+ * Format a date string as a relative time, e.g. "3 weeks ago".
+ * @param date The date string to format
+ * @param now Reference point, for testing
+ * @returns A relative time string, or "" if the date is unparseable
+ */
+export function formatRelativeTime(date: string, now: Date = new Date()): string {
+  const seconds = (new Date(date).getTime() - now.getTime()) / 1000;
+  if (Number.isNaN(seconds)) return '';
+  const [unit, perUnit] =
+    RELATIVE_UNITS.find(([, s]) => Math.abs(seconds) >= s) ??
+    RELATIVE_UNITS[RELATIVE_UNITS.length - 1];
+  return new Intl.RelativeTimeFormat('en', { numeric: 'auto' }).format(
+    Math.round(seconds / perUnit),
+    unit
+  );
+}
+
 /**
  * Format a date string into a human-readable format with optional time
  * @param date The date string to format