Date

The Date is another built-in object in JavaScript. We used it in the development of the SleepTime App. Here is a summary of its operations. For a more detailed reference, visit MDN web docs.

Constructing Dates

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

Caution: Dates are converted to numbers in arithmetic expressions.

  • 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" are optional.

const brendanEichsBirthday = new Date(1961, 6 /* July */, 4);
console.log(brendanEichsBirthday);

Caution: Out-of-range zeroBasedMonth, day, hours, etc. silently roll.

const invalidDate = new Date(2019, 13, -2); // 2020-01-28
console.log(invalidDate);
  • For UTC, construct with date string in format YYYY-MM-DDTHH:mm:ss.sssZ (with literal T, Z):
const firstMillennialUTCnoon = new Date('2000-01-01T12:00:00.000Z');
console.log(firstMillennialUTCnoon);

Caution: Other string formats may be accepted by the constructor, but their format is not standardized.

Caution: Date(...) without new constructs string (in non-standard format).

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

Static Date Functions

  • Date.now() is the current date/time, but it yields milliseconds, not a Date object.
const now = Date.now();
console.log(now);
  • A very useful 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 alexBirthday = Date.parse("May 1, 1980");
console.log(alexBirthday);

alexBirthday = new Date(alexBirthday);
console.log(alexBirthday);

Caution: support for dateString in YYYY-MM-DDTHH:mm:ss.sssZ format is guaranteed. Support for other formats are implementation-dependent.

  • Another useful 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 traditional getter/setter methods getHours/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 alexBirthday = new Date();
alexBirthday.setYear(1985);
alexBirthday.setMonth(5);    // May
alexBirthday.setDate(1);    
console.log(alexBirthday);
  • For UTC, there are UTC variants getUTCFullYear, setUTCFullYear, getUTCMonth, setUTCMonth, and so on.

Date Formatting

  • toISOString yields string in YYYY-MM-DDTHH:mm:ss.sssZ format.
let now = new Date();
console.log(now.toISOString());
  • toString, toDateString, toTimeString yield "humanly readable" string in local time zone, or only the date/time portion:
let now = new Date();
console.log(now.toString());
console.log(now.toDateString());
console.log(now.toTimeString());
  • toUTCString yields "humanly readable" string in UTC:
let now = new Date();
console.log(now.toUTCString());
  • toLocaleString, toLocaleDateString, toLocaleTimeString yield localized string:
let now = new Date();
console.log(now.toLocaleTimeString("en-US"));
console.log(now.toLocaleTimeString("zh-Hans-CN-u-nu-hanidec"));