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.
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,millisecondsThe parameters must be provided in order, and everything starting from “day” are optional.
const brendanEichsBirthday = new Date(1961, 6 /* July */, 4);
console.log(brendanEichsBirthday); // Tue Jul 04 1961 00:00:00 GMT-0400 (Eastern Daylight Time)Caution: Out-of-range zeroBasedMonth, day, hours, etc. silently roll.
const invalidDate = new Date(2019, 13, -2);
console.log(invalidDate); // Wed Jan 29 2020 00:00:00 GMT-0500 (Eastern Standard Time)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); // Sat Jan 01 2000 07:00:00 GMT-0500 (Eastern Standard Time)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);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 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)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.
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 aliBirthday = new Date();
aliBirthday.setYear(1985);
aliBirthday.setMonth(4); // May
aliBirthday.setDate(9);
console.log(aliBirthday);For UTC, there are UTC variants getUTCFullYear, setUTCFullYear, getUTCMonth, setUTCMonth, and so on.
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"));