JS: Date Constructor

new Date()

new Date()

Return a date object that represents the current datetime when this function is called.

console.log(new Date());
// 2026-02-03T22:36:36.626Z

console.log((new Date()).toString());
// Tue Feb 03 2026 14:36:36 GMT-0800 (Pacific Standard Time)

new Date(year, month …)

new Date(year, month)

Return a date object of given local year month.

🛑 WARNING: January is 0.

// note: January is 0
console.log((new Date(2026, 0)));
// 2026-01-01T08:00:00.000Z

new Date(year, month, day, hours, minutes, seconds, ms)

The year, month are required, other parameters are optional.

🛑 WARNING: January is 0.

// note: January is 0
console.log((new Date(2026, 0, 1)));
// 2026-01-01T08:00:00.000Z

The arguments are interpreted as local time. (If you want args to be UTC, use Date.UTC . )

Argument format:

• year → 4 digits integer.
• month → 0 to 11. January is 0.
• day → 1 to 31.
• hours → 0 to 23.
• minutes → 0 to 59.
• seconds → 0 to 59.
• ms → 0 to 999.

new Date(milliseconds)

new Date(milliseconds)

Return a date object that represents the datetime of milliseconds since 01 January, 1970 UTC.

console.log(new Date(0));
// 1970-01-01T00:00:00.000Z

This milliseconds is how JavaScript represents datetime internally.

new Date(string)

new Date(ISO_string)

Return a date object that represents the datetime in ISO_string.

console.log(new Date("2016-04-08T15:48:22.000Z"));
// 2016-04-08T15:48:22.000Z

For the format of ISO_string, see Date.parse

Date() ❌

Date()

• Return a string that represents the current time when this function is called.
• 🛑 WARNING: Arguments to Date() are ignored.

🟢 TIP: don't use this. if you want a date in string format, use (new Date()).toISOString().

console.log(Date());
// Wed Feb 04 2026 16:34:40 GMT-0800 (Pacific Standard Time)

console.assert(typeof (Date()) === "string");
console.assert(Date() === (new Date()).toString());

// arg to Date() are ignored
console.assert(Date(1999) === Date());