Date Object

Date is another built-in object in JavaScript. The sections below go over what you can do with it.

Constructing Dates

Date is measured in milliseconds since the epoch (midnight of January 1, 1970 UTC).
```
let now = new Date(); // yields current date/time.
console.log(now);
```
You can pass a number of milliseconds to the constructor. The valid range is ±100,000,000 days from the epoch.
```
// new Date(milliseconds) constructs Date from milliseconds.
console.log(new Date(10 ** 12));
```

You can pass the following parameters to Date() constructor to create a specific date:
- year,
- zeroBasedMonth,
- day,
- hours,
- minutes,
- seconds,
- milliseconds

The parameters must be provided in order, and everything starting from day is optional.

const brendanEichsBirthday = new Date(1961, 6 /* July */, 4);
console.log(brendanEichsBirthday); // Tue Jul 04 1961 00:00:00 GMT-0400 (Eastern Daylight Time)

const invalidDate = new Date(2019, 13, -2);
console.log(invalidDate); // Wed Jan 29 2020 00:00:00 GMT-0500 (Eastern Standard Time)

To construct a date in UTC, pass a date string in the format YYYY-MM-DDTHH:mm:ss.sssZ, with a literal T and a literal Z:

const firstMillennialUTCnoon = new Date("2000-01-01T12:00:00.000Z");
console.log(firstMillennialUTCnoon); // Sat Jan 01 2000 07:00:00 GMT-0500 (Eastern Standard Time)

const now = Date();
console.log(now);

Static Date Functions

Date.now() yields the current time in milliseconds, not a Date object.
```
const now = Date.now();
console.log(now);
```

Another static function is Date.parse(dateString). It also yields milliseconds, but you can pass that to the Date constructor to get a Date object.

let aliBirthday = Date.parse("May 9, 1985");
console.log(aliBirthday); // 484459200000

aliBirthday = new Date(aliBirthday);
console.log(aliBirthday); // Thu May 09 1985 00:00:00 GMT-0400 (Eastern Daylight Time)

Another static function is Date.UTC(year, zeroBasedMonth, day, hours, minutes, seconds, milliseconds) where arguments after year are optional.

Getters and Setters

The Date object has the usual getter and setter methods, such as getHours and setHours.

const now = new Date();
console.log(now.getFullYear());
console.log(now.getMonth()); // 0-11
console.log(now.getDate()); // 1-31
console.log(now.getHours()); // 0-23
console.log(now.getMinutes());
console.log(now.getSeconds());
console.log(now.getMilliseconds());

let aliBirthday = new Date();
aliBirthday.setFullYear(1985);
aliBirthday.setMonth(4); // May
aliBirthday.setDate(9);
console.log(aliBirthday);

For UTC, there are UTC variants getUTCFullYear, setUTCFullYear, getUTCMonth, setUTCMonth, and so on.

Date Formatting

toISOString yields a string in the YYYY-MM-DDTHH:mm:ss.sssZ format.
```
let now = new Date();
console.log(now.toISOString());
```
toString, toDateString, and toTimeString yield a human-readable string in the local time zone, or only the date or time portion of it:
```
let now = new Date();
console.log(now.toString());
console.log(now.toDateString());
console.log(now.toTimeString());
```

toUTCString yields a human-readable string in UTC:

let now = new Date();
console.log(now.toUTCString());

toLocaleString, toLocaleDateString, and toLocaleTimeString yield a localized string:

let now = new Date();
console.log(now.toLocaleTimeString("en-US"));
console.log(now.toLocaleTimeString("zh-Hans-CN-u-nu-hanidec"));