JavaScript · Lesson 18 / 32 · English

JavaScript Dates

Understand the idea. Change the code. Build confidence with one focused experiment.

Detailed notesRunnable JavaScriptPractice + answer

01 / Understand

Start with the idea

A Date represents an instant in time. Local display depends on the environment, so use explicit UTC values when you need a predictable example.

Learning goal: Explain what the example does, predict its result, and change it to solve the practice task.

02 / Explore

Step-by-step lesson notes

Date Object in JavaScript

The Date object is used to work with dates and times. Internally it stores time as the number of milliseconds since January 1, 1970, 00:00:00 UTC (Unix epoch).

Creating Dates

JavaScript reference fragmentCODE

const now = new Date();                    // current date & time
const specific = new Date("2024-01-15");   // from ISO string
const fromParts = new Date(2024, 0, 15);   // year, month(0-indexed), day
const fromMs = new Date(0);               // Unix epoch: Jan 1, 1970

console.log(Date.now()); // current time as milliseconds

Getting Date Components

JavaScript reference fragmentCODE

const d = new Date();

d.getFullYear();   // 2024
d.getMonth();      // 0-11 (0 = January!)
d.getDate();       // 1-31 (day of month)
d.getDay();        // 0-6 (0 = Sunday)
d.getHours();      // 0-23
d.getMinutes();    // 0-59
d.getSeconds();    // 0-59
d.getMilliseconds(); // 0-999
d.getTime();       // milliseconds since epoch

Watch out! Months are 0-indexed: January = 0, December = 11.

Formatting Dates

JavaScript reference fragmentCODE

const d = new Date();

d.toISOString();        // "2024-01-15T10:30:00.000Z"
d.toLocaleDateString(); // "1/15/2024" (locale-dependent)
d.toLocaleTimeString(); // "10:30:00 AM"
d.toLocaleString();     // "1/15/2024, 10:30:00 AM"

// Custom formatting with Intl.DateTimeFormat
const formatter = new Intl.DateTimeFormat("en-US", {
  year: "numeric",
  month: "long",
  day: "numeric"
});
console.log(formatter.format(d)); // "January 15, 2024"

Date Arithmetic

JavaScript reference fragmentCODE

const start = new Date("2024-01-01");
const end = new Date("2024-12-31");

// Difference in days
const diffMs = end - start; // milliseconds
const diffDays = diffMs / (1000 * 60 * 60 * 24);
console.log(diffDays); // 365

// Add 7 days
const nextWeek = new Date();
nextWeek.setDate(nextWeek.getDate() + 7);

03 / Read & predict

A complete example

Run this complete index.js program in the JavaScript console editor below. The earlier reference fragments may need a browser page or other surrounding setup.

index.js · complete exampleCODE

const lessonDate = new Date("2026-01-15T10:00:00Z");
console.log(lessonDate.toISOString());
console.log(lessonDate.getUTCFullYear());
console.log(lessonDate.getUTCMonth() + 1);

Predict first. Reveal the expected result.

2026-01-15T10:00:00.000Z
2026
1

04 / Experiment

Make one change. See what happens.

Load the example, change a value, predict the output, and run it again. Use the console output to check your reasoning.

Open editor ↗

Wait for the editor to load, then load this example and press Run.

Powered by the free OneCompiler editor. Code you run is sent to that external service. Keep personal data and secret keys out of your examples.

05 / Your turn

Put the idea to work

Practice challenge

Change the timestamp to 2026-02-01T00:00:00Z.

Reveal the answer and reasoning

The UTC month plus one becomes 2 and the ISO output is 2026-02-01T00:00:00.000Z.

One mistake to avoid: getMonth and getUTCMonth are zero-based. Avoid ambiguous date strings such as 01/02/2026.

Your learning checkpoint

Explain the example in your own words.
Complete the challenge before revealing its answer.
Describe the common mistake and how to avoid it.