diff --git a/site/countdown/index.html b/site/countdown/index.html
new file mode 100644
index 0000000..02c52e4
--- /dev/null
+++ b/site/countdown/index.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <meta name="color-scheme" content="light dark">
+  <title>Countdown Timer - Tools</title>
+  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/@picocss/pico@2/css/pico.min.css">
+  <link rel="stylesheet" href="style.css">
+  <script type="module" src="main.js"></script>
+</head>
+<body class="container">
+  <header>
+    <hgroup>
+      <h1>Countdown Timer</h1>
+      <p>Set a time and count down</p>
+    </hgroup>
+  </header>
+  <main>
+    <section class="timer" aria-label="Countdown timer">
+      <div class="counter" role="group" aria-label="Time remaining">
+        <input id="minutes" type="text" inputmode="numeric" pattern="[0-9]*"
+          value="0" aria-label="Minutes" autocomplete="off" spellcheck="false"
+          title="Up/Down: change by 1. Shift+Up/Down: change by 10.">
+        <span aria-hidden="true">:</span>
+        <input id="seconds" type="text" inputmode="numeric" pattern="[0-9]*"
+          value="00" aria-label="Seconds" autocomplete="off" spellcheck="false"
+          title="Up/Down: change by 1. Shift+Up/Down: change by 10.">
+      </div>
+      <div class="controls">
+        <button id="start-stop" type="button">Start</button>
+        <button id="reset" class="secondary" type="button">Reset</button>
+      </div>
+      <span id="status" class="visually-hidden" role="status">Idle</span>
+    </section>
+  </main>
+</body>
+</html>
diff --git a/site/countdown/main.js b/site/countdown/main.js
new file mode 100644
index 0000000..9c99b30
--- /dev/null
+++ b/site/countdown/main.js
@@ -0,0 +1,94 @@
+import { Countdown, integer } from './timer.mjs';
+
+const timer = new Countdown();
+const minutes = document.querySelector('#minutes');
+const seconds = document.querySelector('#seconds');
+const counter = document.querySelector('.counter');
+const startStop = document.querySelector('#start-stop');
+const reset = document.querySelector('#reset');
+const status = document.querySelector('#status');
+const fields = [minutes, seconds];
+let ticker;
+
+function resize(field) {
+    field.style.width = `${Math.max(field === seconds ? 2 : 1, field.value.length)}ch`;
+    field.dataset.valid = field.value;
+}
+
+function render(updateFields = true) {
+    if (updateFields) {
+        const display = timer.display();
+        minutes.value = display.minutes;
+        seconds.value = display.seconds;
+    }
+    fields.forEach(field => {
+        field.readOnly = timer.state === 'running';
+        resize(field);
+    });
+    counter.classList.toggle('finished', timer.finished);
+    startStop.textContent = timer.state === 'running' ? 'Stop' : 'Start';
+    const message = timer.finished ? 'Countdown finished' :
+        timer.state === 'running' ? 'Running' :
+        timer.state === 'paused' ? 'Paused' : 'Idle';
+    if (status.textContent !== message) status.textContent = message;
+    if (timer.state !== 'running') {
+        clearInterval(ticker);
+        ticker = undefined;
+    }
+}
+
+function edited() {
+    timer.edit(minutes.value, seconds.value);
+    render(false);
+}
+
+for (const field of fields) {
+    resize(field);
+    field.addEventListener('input', () => {
+        if (field.readOnly || !/^\d*$/.test(field.value)) {
+            field.value = field.dataset.valid;
+            return;
+        }
+        edited();
+    });
+    field.addEventListener('keydown', event => {
+        if (field.readOnly || !['ArrowUp', 'ArrowDown'].includes(event.key)) return;
+        event.preventDefault();
+        const step = event.shiftKey ? 10n : 1n;
+        const value = integer(field.value) + (event.key === 'ArrowUp' ? step : -step);
+        field.value = String(value < 0n ? 0n : value);
+        edited();
+    });
+    field.addEventListener('blur', () => {
+        if (field.readOnly) return;
+        field.value = String(integer(field.value));
+        if (field === seconds) field.value = field.value.padStart(2, '0');
+        resize(field);
+    });
+}
+
+startStop.addEventListener('click', () => {
+    timer.toggle();
+    render();
+    if (timer.state === 'running' && ticker === undefined) {
+        // This is only a display refresh; elapsed time comes from the deadline.
+        ticker = setInterval(() => {
+            timer.update();
+            render();
+        }, 50);
+    }
+});
+
+reset.addEventListener('click', () => {
+    timer.reset();
+    render();
+});
+
+document.addEventListener('visibilitychange', () => {
+    if (timer.state === 'running') {
+        timer.update();
+        render();
+    }
+});
+
+render();
diff --git a/site/countdown/style.css b/site/countdown/style.css
new file mode 100644
index 0000000..cab1ff0
--- /dev/null
+++ b/site/countdown/style.css
@@ -0,0 +1,101 @@
+body {
+    min-height: 100vh;
+    min-height: 100dvh;
+    display: flex;
+    flex-direction: column;
+}
+
+/* Keep the counter vertically centered in the viewport, below the header. */
+header {
+    position: absolute;
+    top: 0;
+}
+
+main {
+    flex: 1;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    padding-block: 10rem;
+}
+
+.timer {
+    text-align: center;
+    min-width: 0;
+    max-width: 100%;
+    margin: 0;
+}
+
+.counter {
+    display: flex;
+    align-items: baseline;
+    justify-content: center;
+    max-width: 100%;
+    overflow-x: auto;
+    font-family: var(--pico-font-family-monospace);
+    font-size: clamp(3rem, 12vw, 6rem);
+    line-height: 1.2;
+    margin-bottom: 1.5rem;
+}
+
+.counter input {
+    box-sizing: content-box;
+    flex: 0 0 auto;
+    width: 1ch;
+    height: auto;
+    margin: 0;
+    padding: 0;
+    border: 0;
+    border-radius: 0;
+    background: transparent;
+    box-shadow: none;
+    color: inherit;
+    font: inherit;
+    line-height: inherit;
+    text-align: right;
+}
+
+.counter #seconds {
+    width: 2ch;
+    text-align: left;
+}
+
+.counter input:focus {
+    background: transparent;
+    box-shadow: none;
+}
+
+.counter input:focus-visible:not([readonly]) {
+    outline: 2px solid var(--pico-primary);
+    outline-offset: 4px;
+}
+
+.controls {
+    display: flex;
+    justify-content: center;
+    gap: 1rem;
+}
+
+.controls button {
+    margin: 0;
+    min-width: 6rem;
+}
+
+.counter.finished {
+    animation: flash 1s steps(1, end) infinite;
+}
+
+@keyframes flash {
+    0%, 100% { opacity: 1; }
+    50% { opacity: 0; }
+}
+
+.visually-hidden {
+    position: absolute;
+    width: 1px;
+    height: 1px;
+    padding: 0;
+    overflow: hidden;
+    clip-path: inset(50%);
+    white-space: nowrap;
+}
diff --git a/site/countdown/timer.mjs b/site/countdown/timer.mjs
new file mode 100644
index 0000000..10d16eb
--- /dev/null
+++ b/site/countdown/timer.mjs
@@ -0,0 +1,64 @@
+// BigInt keeps entered integers exact, even for very large countdowns.
+export function integer(value) {
+    return /^\d+$/.test(String(value)) ? BigInt(value) : 0n;
+}
+
+export function splitSeconds(seconds) {
+    return {
+        minutes: String(seconds / 60n),
+        seconds: String(seconds % 60n).padStart(2, '0'),
+    };
+}
+
+export class Countdown {
+    constructor(now = Date.now) {
+        this.now = now;
+        this.state = 'idle';
+        this.finished = false;
+        this.duration = 0n;
+        this.remaining = 0n;
+        this.deadline = 0n;
+    }
+
+    edit(minutes, seconds) {
+        if (this.state === 'running') return;
+        this.duration = (integer(minutes) * 60n + integer(seconds)) * 1000n;
+        this.remaining = this.duration;
+        this.state = 'idle';
+        this.finished = false;
+    }
+
+    update() {
+        if (this.state !== 'running') return;
+        const delta = this.deadline - BigInt(this.now());
+        this.remaining = delta > 0n ? delta : 0n;
+        if (this.remaining === 0n) {
+            this.state = 'idle';
+            this.finished = true;
+        }
+    }
+
+    toggle() {
+        if (this.state === 'running') {
+            this.update();
+            if (!this.finished) this.state = 'paused';
+            return;
+        }
+        if (this.finished) this.remaining = this.duration;
+        this.finished = false;
+        this.deadline = BigInt(this.now()) + this.remaining;
+        this.state = 'running';
+        this.update();
+    }
+
+    reset() {
+        this.remaining = this.duration;
+        this.state = 'idle';
+        this.finished = false;
+    }
+
+    display() {
+        // Round up so the timer only displays zero once its deadline is reached.
+        return splitSeconds((this.remaining + 999n) / 1000n);
+    }
+}
diff --git a/site/countdown/timer.test.mjs b/site/countdown/timer.test.mjs
new file mode 100644
index 0000000..fce24f7
--- /dev/null
+++ b/site/countdown/timer.test.mjs
@@ -0,0 +1,142 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { Countdown, integer, splitSeconds } from './timer.mjs';
+
+function fixture() {
+    let now = 1000;
+    return {
+        timer: new Countdown(() => now),
+        advance(ms) { now += ms; },
+    };
+}
+
+test('starts idle at zero and accepts nonnegative integers only', () => {
+    const { timer } = fixture();
+    assert.equal(timer.state, 'idle');
+    assert.deepEqual(timer.display(), { minutes: '0', seconds: '00' });
+    for (const invalid of ['-1', '1.5', '1e2', 'abc', '']) {
+        assert.equal(integer(invalid), 0n);
+    }
+    assert.equal(integer('00090'), 90n);
+});
+
+test('normalizes overflowing seconds and preserves arbitrarily large integers', () => {
+    const { timer } = fixture();
+    timer.edit('0', '90');
+    timer.toggle();
+    assert.deepEqual(timer.display(), { minutes: '1', seconds: '30' });
+    const minutes = '123456789012345678901234567890';
+    timer.reset();
+    timer.edit(minutes, '120');
+    assert.deepEqual(timer.display(), { minutes: String(BigInt(minutes) + 2n), seconds: '00' });
+    assert.deepEqual(splitSeconds(61n), { minutes: '1', seconds: '01' });
+});
+
+test('uses elapsed time, rounding up, even when refreshes are delayed', () => {
+    const { timer, advance } = fixture();
+    timer.edit('0', '90');
+    timer.toggle();
+    advance(999);
+    timer.update();
+    assert.deepEqual(timer.display(), { minutes: '1', seconds: '30' });
+    advance(1);
+    timer.update();
+    assert.deepEqual(timer.display(), { minutes: '1', seconds: '29' });
+    advance(75000);
+    timer.update();
+    assert.deepEqual(timer.display(), { minutes: '0', seconds: '14' });
+});
+
+test('pause/resume preserves fractional seconds and excludes paused time', () => {
+    const { timer, advance } = fixture();
+    timer.edit('0', '10');
+    timer.toggle();
+    advance(1250);
+    timer.toggle();
+    assert.equal(timer.state, 'paused');
+    assert.equal(timer.remaining, 8750n);
+    advance(100000);
+    timer.update();
+    assert.equal(timer.remaining, 8750n);
+    timer.toggle();
+    advance(749);
+    timer.update();
+    assert.equal(timer.display().seconds, '09');
+    advance(1);
+    timer.update();
+    assert.equal(timer.display().seconds, '08');
+});
+
+test('reset restores normalized last input while running or paused', () => {
+    const { timer, advance } = fixture();
+    timer.edit('2', '90');
+    for (const pause of [false, true]) {
+        timer.toggle();
+        advance(1500);
+        if (pause) timer.toggle();
+        timer.reset();
+        assert.equal(timer.state, 'idle');
+        assert.deepEqual(timer.display(), { minutes: '3', seconds: '30' });
+        assert.equal(timer.finished, false);
+    }
+});
+
+test('editing a paused timer establishes a new countdown and reset value', () => {
+    const { timer, advance } = fixture();
+    timer.edit('0', '10');
+    timer.toggle();
+    advance(2000);
+    timer.toggle();
+    timer.edit('0', '90');
+    assert.equal(timer.state, 'idle');
+    timer.toggle();
+    advance(3000);
+    timer.reset();
+    assert.deepEqual(timer.display(), { minutes: '1', seconds: '30' });
+});
+
+test('running countdown cannot be edited', () => {
+    const { timer } = fixture();
+    timer.edit('0', '10');
+    timer.toggle();
+    timer.edit('5', '0');
+    assert.equal(timer.duration, 10000n);
+    assert.equal(timer.state, 'running');
+});
+
+test('completion returns to idle and Start restarts the original duration', () => {
+    const { timer, advance } = fixture();
+    timer.edit('0', '2');
+    timer.toggle();
+    advance(5000);
+    timer.update();
+    assert.equal(timer.state, 'idle');
+    assert.equal(timer.finished, true);
+    assert.equal(timer.display().seconds, '00');
+    timer.toggle();
+    assert.equal(timer.finished, false);
+    assert.equal(timer.state, 'running');
+    assert.equal(timer.display().seconds, '02');
+});
+
+test('editing or resetting after completion clears the completion flag', () => {
+    const { timer } = fixture();
+    timer.toggle();
+    assert.equal(timer.finished, true);
+    timer.edit('0', '3');
+    assert.equal(timer.finished, false);
+    timer.toggle();
+    timer.reset();
+    assert.equal(timer.finished, false);
+    assert.equal(timer.display().seconds, '03');
+});
+
+test('stopping after the deadline completes instead of leaving a paused zero', () => {
+    const { timer, advance } = fixture();
+    timer.edit('0', '1');
+    timer.toggle();
+    advance(1000);
+    timer.toggle();
+    assert.equal(timer.state, 'idle');
+    assert.equal(timer.finished, true);
+});
diff --git a/site/index.html b/site/index.html
index 0c17283..bfb00c4 100644
--- a/site/index.html
+++ b/site/index.html
@@ -19,6 +19,7 @@
   <main>
     <ul>
       <li><a href="/clocks/">Clocks</a></li>
+      <li><a href="/countdown/">Countdown Timer</a></li>
       <li><a href="/freelens-logo/">Freelens Logo Maker</a></li>
       <li><a href="/golines/">Golines Playground</a></li>
       <li><a href="/gotemplate/">Go Template Playground</a></li>