JavaScript Time Difference: Compute Elapsed Time with Explicit Offsets

The task

Compare a booking start and end across midnight in a supplied +02:00 offset. Keep that offset explicit so the result does not depend on the computer’s local time zone.

Runnable Node 22 example

const assert = require('node:assert/strict');

const start = Date.parse('2026-10-20T23:30:00+02:00');
const end = Date.parse('2026-10-21T01:00:00+02:00');
assert(Number.isFinite(start) && Number.isFinite(end));
const milliseconds = end - start;
const hours = milliseconds / 3_600_000;
assert.equal(milliseconds, 5_400_000);
assert.equal(hours, 1.5);
assert.equal(Date.parse('2026-10-20T21:30:00Z'), start);
assert.equal(end - (start + 86_400_000), -81_000_000);
console.log(`milliseconds=${milliseconds}; hours=${hours}`);

Expected output and explanation

The interval spans ninety minutes, even though the displayed local calendar date changes. The UTC-equivalent assertion confirms that an explicit offset identifies the same starting instant, rather than creating a different duration.

A fixed twenty-four-hour interval is 86,400,000 milliseconds. That number should not be silently substituted for “next calendar day” in scheduling logic. Calendar operations require a chosen time zone and date policy, especially where offset changes can occur. This fixture avoids that ambiguity by supplying two explicit instants.

Pitfall to check

Check parsing before subtracting, and decide what a negative duration means for your application. Do not apply Math.abs automatically if an end-before-start input should be rejected. Duration calculations cannot validate that a booking was approved.