diff --git a/extension/src/shared/__tests__/relative-time.test.ts b/extension/src/shared/__tests__/relative-time.test.ts
new file mode 100644
index 0000000..7b4d92e
--- /dev/null
+++ b/extension/src/shared/__tests__/relative-time.test.ts
@@ -0,0 +1,46 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { relativeTime, daysUntilPurge } from '../relative-time';
+
+const NOW_UNIX = 1779552000; // fixed reference instant
+
+beforeEach(() => {
+  vi.useFakeTimers();
+  vi.setSystemTime(new Date(NOW_UNIX * 1000));
+});
+
+afterEach(() => {
+  vi.useRealTimers();
+});
+
+describe('relativeTime', () => {
+  it('returns "just now" under 60s', () => {
+    expect(relativeTime(NOW_UNIX - 30)).toBe('just now');
+  });
+  it('returns minutes under an hour', () => {
+    expect(relativeTime(NOW_UNIX - 600)).toBe('10m ago');
+  });
+  it('returns hours under a day', () => {
+    expect(relativeTime(NOW_UNIX - 7200)).toBe('2h ago');
+  });
+  it('returns days under a week', () => {
+    expect(relativeTime(NOW_UNIX - 3 * 86400)).toBe('3d ago');
+  });
+  it('returns weeks under a month', () => {
+    expect(relativeTime(NOW_UNIX - 14 * 86400)).toBe('2w ago');
+  });
+  it('returns months above 30 days', () => {
+    expect(relativeTime(NOW_UNIX - 90 * 86400)).toBe('3mo ago');
+  });
+});
+
+describe('daysUntilPurge', () => {
+  it('returns null for forever retention', () => {
+    expect(daysUntilPurge(NOW_UNIX - 5 * 86400, { kind: 'forever' })).toBeNull();
+  });
+  it('returns remaining days for a recent trash', () => {
+    expect(daysUntilPurge(NOW_UNIX - 8 * 86400, { kind: 'days', value: 30 })).toBe(22);
+  });
+  it('clamps to zero when retention already elapsed', () => {
+    expect(daysUntilPurge(NOW_UNIX - 60 * 86400, { kind: 'days', value: 30 })).toBe(0);
+  });
+});
diff --git a/extension/src/shared/relative-time.ts b/extension/src/shared/relative-time.ts
new file mode 100644
index 0000000..f7ccc93
--- /dev/null
+++ b/extension/src/shared/relative-time.ts
@@ -0,0 +1,27 @@
+/// Single source of truth for relative-time formatting and trash-retention math.
+/// Pulled out of five near-duplicate inline copies (settings-vault, devices,
+/// trash, field-history, vault/vault).
+
+import type { TrashRetention } from './types';
+
+/// Format a past unix timestamp (seconds) as "Nm ago" / "Nh ago" / "Nd ago" /
+/// "Nw ago" / "Nmo ago" relative to now. Returns "just now" under 60 seconds.
+export function relativeTime(unixSec: number): string {
+  const now = Math.floor(Date.now() / 1000);
+  const diff = now - unixSec;
+  if (diff < 60)      return 'just now';
+  if (diff < 3600)    return `${Math.floor(diff / 60)}m ago`;
+  if (diff < 86400)   return `${Math.floor(diff / 3600)}h ago`;
+  if (diff < 604800)  return `${Math.floor(diff / 86400)}d ago`;
+  if (diff < 2592000) return `${Math.floor(diff / 604800)}w ago`;
+  return `${Math.floor(diff / 2592000)}mo ago`;
+}
+
+/// Days remaining until an item trashed at `trashedAt` (unix seconds) will be
+/// auto-purged given the vault's retention policy. Returns null for forever
+/// retention; clamps to 0 when the retention window has already elapsed.
+export function daysUntilPurge(trashedAt: number, retention: TrashRetention): number | null {
+  if (retention.kind === 'forever') return null;
+  const trashedDaysAgo = Math.floor((Date.now() / 1000 - trashedAt) / 86400);
+  return Math.max(0, retention.value - trashedDaysAgo);
+}