diff --git a/site/countdown/main.js b/site/countdown/main.js
index 9c99b30..c51aa60 100644
--- a/site/countdown/main.js
+++ b/site/countdown/main.js
@@ -1,4 +1,4 @@
-import { Countdown, integer } from './timer.mjs';
+import { Countdown, integer, adjustTime } from './timer.mjs';
 
 const timer = new Countdown();
 const minutes = document.querySelector('#minutes');
@@ -55,8 +55,10 @@ for (const field of fields) {
         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);
+        const adjusted = adjustTime(minutes.value, seconds.value, field.id,
+            event.key === 'ArrowUp' ? step : -step);
+        minutes.value = adjusted.minutes;
+        seconds.value = adjusted.seconds;
         edited();
     });
     field.addEventListener('blur', () => {
diff --git a/site/countdown/timer.mjs b/site/countdown/timer.mjs
index 10d16eb..44e95c2 100644
--- a/site/countdown/timer.mjs
+++ b/site/countdown/timer.mjs
@@ -10,6 +10,16 @@ export function splitSeconds(seconds) {
     };
 }
 
+export function adjustTime(minutes, seconds, field, delta) {
+    const total = integer(minutes) * 60n + integer(seconds);
+    if (field === 'minutes') {
+        const adjusted = total / 60n + delta;
+        return splitSeconds((adjusted < 0n ? 0n : adjusted) * 60n + total % 60n);
+    }
+    const adjusted = total + delta;
+    return splitSeconds(adjusted < 0n ? 0n : adjusted);
+}
+
 export class Countdown {
     constructor(now = Date.now) {
         this.now = now;
diff --git a/site/countdown/timer.test.mjs b/site/countdown/timer.test.mjs
index fce24f7..bed739f 100644
--- a/site/countdown/timer.test.mjs
+++ b/site/countdown/timer.test.mjs
@@ -1,6 +1,38 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { Countdown, integer, splitSeconds } from './timer.mjs';
+import { Countdown, integer, splitSeconds, adjustTime } from './timer.mjs';
+
+for (const [minutes, seconds, delta, expectedMinutes, expectedSeconds] of [
+    ['1', '59', 1n, '2', '00'],
+    ['2', '00', -1n, '1', '59'],
+    ['2', '05', -10n, '1', '55'],
+    ['1', '58', 10n, '2', '08'],
+    ['2', '00', -10n, '1', '50'],
+    ['0', '90', 1n, '1', '31'],
+    ['0', '90', -10n, '1', '20'],
+    ['0', '00', -1n, '0', '00'],
+    ['0', '05', -10n, '0', '00'],
+]) {
+    test(`seconds adjustment ${minutes}:${seconds} ${delta} normalizes and rolls`, () => {
+        assert.deepEqual(adjustTime(minutes, seconds, 'seconds', delta), {
+            minutes: expectedMinutes,
+            seconds: expectedSeconds,
+        });
+    });
+}
+
+test('minute adjustments normalize first and keep minutes nonnegative', () => {
+    assert.deepEqual(adjustTime('0', '90', 'minutes', 1n), { minutes: '2', seconds: '30' });
+    assert.deepEqual(adjustTime('0', '90', 'minutes', -1n), { minutes: '0', seconds: '30' });
+    assert.deepEqual(adjustTime('1', '90', 'minutes', 10n), { minutes: '12', seconds: '30' });
+    assert.deepEqual(adjustTime('1', '90', 'minutes', -10n), { minutes: '0', seconds: '30' });
+});
+
+test('keyboard adjustments preserve large integer precision', () => {
+    assert.deepEqual(adjustTime('12345678901234567890', '59', 'seconds', 1n), {
+        minutes: '12345678901234567891', seconds: '00',
+    });
+});
 
 function fixture() {
     let now = 1000;