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JavaScript Dates

JavaScript Dates

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The Date object represents a single moment in time.

A date is stored as the number of milliseconds since the start of 1970.

Everything else is a way of reading or displaying that number.

Example

Example

javascript

const date = new Date(2026, 0, 15);

console.log(date.getFullYear());
console.log(date.getDate());

The output is 2026 then 15.

Creating a Date

new Date() with no arguments gives the current moment.

new Date(year, month, day) builds a specific date.

new Date("2026-01-15") parses an ISO string.

Syntax

Syntax

javascript

new Date(year, month, day);

Months start at 0, which is the classic trap.

Months Start at Zero

January is 0 and December is 11.

Days of the month, confusingly, start at 1.

Example

Example

javascript

const january = new Date(2026, 0, 15);
const december = new Date(2026, 11, 15);

console.log(january.getMonth());
console.log(december.getMonth());
console.log(january.getDate());

The output is 0, 11, 15.

Parsing a String

The ISO format is the only one that behaves consistently everywhere.

Note that an ISO date string is treated as UTC.

Example

Example

javascript

const date = new Date("2026-01-15T00:00:00Z");

console.log(date.getUTCFullYear());
console.log(date.getUTCMonth());
console.log(date.getUTCDate());

Using the UTC readers avoids any timezone surprise.

Timestamps

getTime gives the underlying number of milliseconds.

Example

Example

javascript

const date = new Date(Date.UTC(2026, 0, 15));

console.log(typeof date.getTime());
console.log(date.getTime() > 0);

Date.now() gives the current timestamp directly.

Reading the Parts

Each part has its own getter.

Example

Example

javascript

const date = new Date(Date.UTC(2026, 0, 15, 14, 30));

console.log(date.getUTCHours());
console.log(date.getUTCMinutes());
console.log(date.getUTCDay());

getUTCDay gives the weekday, where 0 is Sunday.

Dates Are Mutable

The setters change the date in place, unlike most modern JavaScript.

Copy it first if the original matters.

Example

Example

javascript

const original = new Date(Date.UTC(2026, 0, 15));
const copy = new Date(original.getTime());

copy.setUTCDate(20);

console.log(original.getUTCDate());
console.log(copy.getUTCDate());

The output is 15 then 20.

Complete Example

Complete Example

html

<!DOCTYPE html>
<html>
<head>
  <title>JavaScript Dates</title>
</head>
<body>

  <h1>JavaScript Dates</h1>

  <p id="out"></p>

  <script>
    const date = new Date(2026, 0, 15);

    document.getElementById("out").innerHTML =
      "Year: " + date.getFullYear() +
      "<br>Month number: " + date.getMonth() + " (January is 0)" +
      "<br>Day of month: " + date.getDate() +
      "<br>Today is: " + new Date().getFullYear();
  </script>

</body>
</html>

Try It Yourself

Run the above example in the Try It Editor.

Try December:

Change the month to 11 and check the number that appears.

Important Points

  • A Date is a moment stored as milliseconds since 1970.
  • Months start at 0; days of the month start at 1.
  • The ISO string format parses consistently.
  • getTime gives the raw timestamp.
  • Dates are mutable, so copy before changing.

Conclusion

The zero-based month is the single biggest source of date bugs.

Working in UTC removes a whole class of timezone confusion.

For anything complex, a dedicated date library is worth the weight.

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