JavaScript Dates
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
javascript
const date = new Date(2026, 0, 15);
console.log(date.getFullYear());
console.log(date.getDate());The output is 2026 then 15.
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
javascript
new Date(year, month, day);Months start at 0, which is the classic trap.
January is 0 and December is 11.
Days of the month, confusingly, start at 1.
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.
The ISO format is the only one that behaves consistently everywhere.
Note that an ISO date string is treated as UTC.
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.
getTime gives the underlying number of milliseconds.
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.
Each part has its own getter.
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.
The setters change the date in place, unlike most modern JavaScript.
Copy it first if the original matters.
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
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>Run the above example in the Try It Editor.
Try December:
Change the month to 11 and check the number that appears.
getTime gives the raw timestamp.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.