diff --git a/src/lib/live-timer.ts b/src/lib/live-timer.ts
new file mode 100644
index 0000000..0d95625
--- /dev/null
+++ b/src/lib/live-timer.ts
@@ -0,0 +1,36 @@
+/**
+ * Server-stamped phase timer math for the grand-finale ceremony.
+ *
+ * The cursor stores `phaseStartedAt` + `phaseDurationSeconds` plus a pause
+ * accumulator; every client derives the countdown locally from those stamps,
+ * so all screens agree and overtime is just a negative remainder.
+ */
+
+export type PhaseTimerState = {
+  phaseStartedAt: Date | string | null
+  phaseDurationSeconds: number | null
+  phasePausedAt: Date | string | null
+  phasePausedAccumMs: number
+}
+
+export function elapsedMs(t: PhaseTimerState, now: Date = new Date()): number {
+  if (!t.phaseStartedAt) return 0
+  const start = new Date(t.phaseStartedAt).getTime()
+  const end = t.phasePausedAt ? new Date(t.phasePausedAt).getTime() : now.getTime()
+  return Math.max(0, end - start - t.phasePausedAccumMs)
+}
+
+/** Seconds left on the phase timer; negative = overtime; null = no timer running. */
+export function remainingSeconds(t: PhaseTimerState, now: Date = new Date()): number | null {
+  if (!t.phaseStartedAt || t.phaseDurationSeconds == null) return null
+  return t.phaseDurationSeconds - Math.floor(elapsedMs(t, now) / 1000)
+}
+
+/** `5:05` for positive seconds, `+1:23` for overtime. */
+export function formatClock(seconds: number): string {
+  const over = seconds < 0
+  const abs = Math.abs(seconds)
+  const m = Math.floor(abs / 60)
+  const s = abs % 60
+  return `${over ? '+' : ''}${m}:${s.toString().padStart(2, '0')}`
+}
diff --git a/tests/unit/live-timer.test.ts b/tests/unit/live-timer.test.ts
new file mode 100644
index 0000000..374ea91
--- /dev/null
+++ b/tests/unit/live-timer.test.ts
@@ -0,0 +1,75 @@
+/**
+ * Server-stamped phase timer math: every screen (admin, jury, big screen)
+ * derives the same countdown from cursor timestamps — no client-local clocks.
+ */
+import { describe, it, expect } from 'vitest'
+import { elapsedMs, remainingSeconds, formatClock } from '@/lib/live-timer'
+
+const t0 = new Date('2026-06-11T10:00:00Z')
+const at = (s: number) => new Date(t0.getTime() + s * 1000)
+
+describe('live-timer', () => {
+  it('elapsedMs counts from phaseStartedAt', () => {
+    expect(
+      elapsedMs(
+        { phaseStartedAt: t0, phaseDurationSeconds: 300, phasePausedAt: null, phasePausedAccumMs: 0 },
+        at(90)
+      )
+    ).toBe(90_000)
+  })
+
+  it('elapsedMs freezes while paused and subtracts accumulated pause', () => {
+    // paused at +60s, asked at +90s → frozen at 60s
+    expect(
+      elapsedMs(
+        { phaseStartedAt: t0, phaseDurationSeconds: 300, phasePausedAt: at(60), phasePausedAccumMs: 0 },
+        at(90)
+      )
+    ).toBe(60_000)
+    // resumed with 30s pause accumulated, asked at +120s → 90s elapsed
+    expect(
+      elapsedMs(
+        { phaseStartedAt: t0, phaseDurationSeconds: 300, phasePausedAt: null, phasePausedAccumMs: 30_000 },
+        at(120)
+      )
+    ).toBe(90_000)
+  })
+
+  it('elapsedMs accepts ISO strings (serialized cursor)', () => {
+    expect(
+      elapsedMs(
+        {
+          phaseStartedAt: t0.toISOString(),
+          phaseDurationSeconds: 300,
+          phasePausedAt: null,
+          phasePausedAccumMs: 0,
+        },
+        at(45)
+      )
+    ).toBe(45_000)
+  })
+
+  it('remainingSeconds goes negative on overtime', () => {
+    expect(
+      remainingSeconds(
+        { phaseStartedAt: t0, phaseDurationSeconds: 60, phasePausedAt: null, phasePausedAccumMs: 0 },
+        at(75)
+      )
+    ).toBe(-15)
+  })
+
+  it('remainingSeconds is null without a running timer', () => {
+    expect(
+      remainingSeconds(
+        { phaseStartedAt: null, phaseDurationSeconds: null, phasePausedAt: null, phasePausedAccumMs: 0 },
+        t0
+      )
+    ).toBeNull()
+  })
+
+  it('formatClock renders m:ss and overtime as +m:ss', () => {
+    expect(formatClock(305)).toBe('5:05')
+    expect(formatClock(0)).toBe('0:00')
+    expect(formatClock(-83)).toBe('+1:23')
+  })
+})