The Date class stores dates as an integer, the "epoch". This number counts the number of seconds that have passed since January 1st, 1970. All operations are done to this number, which is converted back to year, month, day, etc. on demand. Also, Dates are stored in UTC (Coordinated Universal Time), and therefore converted to local time on demand as well.

Creating a Date

Creating dates from strings sets them as UTC time:

new Date(); // current computer time
new Date("2024-03-05");
new Date("2024-03-05 05:55:41.123");

Creating dates from integers means converting them from local time:

new Date(2024, 0, 1); // '2023-12-31 23:00:00' since we are GMT+1

Copying dates can be done by passing them in the constructor:

const d = new Date(`2000-01-01`);
const e = new Date(d); // copy of 'd'

Methods to get parts of a Date

There are methods to get the same parts but for the UTC date: getUTCDate, getUTCDay, getUTCHours, etc.

Time intervals

When using Date as numbers (in subtractions or comparisons), they are autoconverted to the underlying number (the Epoch), so it is safe to subtract Dates and compare them.

const start = new Date(`2024-01-01`);
const end = new Date(`2024-02-13`);
if (end < start) {
    throw new Error(`The end date is before the start!`);
}

const MINUTE = 60 * 1000;
const HOUR = 60 * MINUTE;
const DAY = 24 * HOUR;

const timeDif = end - start; // equivalent end.getTime() - start.getTime()
const days = timeDif / DAY;
const hours = (timeDif % DAY) / HOUR;
console.log(`${days} days and ${hours} hours`);

Adding or subtracting time

Instead of having to use milliseconds for operations like adding or subtracting time periods from a Date, we can use another property of Dates: if you set any part of a date to a value out of bounds, its value will be autocorrected.

new Date(2024, 0, 52) // -> 2024-02-20T23:00:00.000Z

So, setting the 52nd of January will actually be the 20th of February. Using this, we can add a certain number of days or hours to a date:

const d = new Date();
d.setDate(d.getDate() + 2); // Two days from now
const e = new Date(d);
e.setHours(e.getHours() + 36); // 36 hours after 'd'