Basics

const today = new Date(); 

console.log(today.toString());
// Fri Mar 10 2023 17:39:33 GMT+1100 (Australian Eastern Daylight Time)

console.log(today); // 2023-03-10T06:39:33.953Z

console.log(today.toISOString()); // 2023-03-10T06:39:33.953Z

const date_2025_jan_1_13_30_15 = new Date(2025, 0, 1, 13, 30, 15);

console.log(date_2025_jan_1_13_30_15); // 2025-01-01T02:30:15.000Z

console.log(typeof today); // 'object'

console.log(today instanceof Date); // true

Formatting Dates

/** Local New Year 2050 */
const date = new Date(2050, 0, 1, 0, 0, 0);

/// Mission: format as YYYY-MM-DD hh:mm:ss

function padZero(num) {
  return num < 10 ? `0${num}` : num.toString();
}

/** date => 'YYYY-MM-DD hh:mm:ss' */
function format(date) {
  const YYYY = date.getFullYear().toString();
  const MM = padZero(date.getMonth() + 1);
  const DD = padZero(date.getDate());
  const hh = padZero(date.getHours());
  const mm = padZero(date.getMinutes());
  const ss = padZero(date.getSeconds());
  return `${YYYY}-${MM}-${DD} ${hh}:${mm}:${ss}`;
}

console.log(format(date));

timestamp

// I'm in UTC+11
const date = new Date(2030, 0, 1, 11);
console.log(date); // 2030-01-01T00:00:00.000Z

const timestamp = date.getTime();
console.log(timestamp); // 1893456000000

const copy = new Date(timestamp);
console.log(copy); // 2030-01-01T00:00:00.000Z

/** Move behind by 24 hours */
const yesterdayTimestamp =
  timestamp - 24 * 60 * 60 * 1000;
const yesterday = new Date(yesterdayTimestamp);
console.log(yesterday); // 2029-12-31T00:00:00.000Z

Timezone

// Hint: we are +11 time zone
// Default inputs are in local time
const date_2025_jan_1_11am_australia = new Date(2025, 0, 1, 11);
// Time is absolute, default representation is UTC
console.log(date_2025_jan_1_11am_australia); // 2025-01-01T00:00:00.000Z

// In local its 11 hours
console.log(date_2025_jan_1_11am_australia.getHours()); // 11
// In UTC its 00 hours 
console.log(date_2025_jan_1_11am_australia.getUTCHours()); // 0

// UTC version
const date_2025_jan_1_midnight_utc = new Date(Date.UTC(2025, 0, 1, 0));
console.log(date_2025_jan_1_midnight_utc); // 2025-01-01T00:00:00.000Z

const minuteOffsetLocalTimeZone = new Date().getTimezoneOffset();
// offset = (UTC - LOCAL) minutes

// We are +11 hours => 11*60 = 660 minutes ahead of UTC
console.log(minuteOffsetLocalTimeZone); // -660

/** Time zone offset formatted as `(+/-)hh:m` */
function formattedTimezone() {
  const offset = new Date().getTimezoneOffset();
  const sign = offset > 0 ? "-" : "+";
  const minutes = Math.abs(offset % 60);
  const hours = Math.floor(Math.abs(offset / 60));
  return `${sign}${hours}:${minutes}`;
}

console.log(formattedTimezone()); // +11:0

now

Example use case of performance testing:

let timestamp;

timestamp = new Date().getTime();

timestamp = Date.now();

console.log(timestamp); // 1678435753900

function measure(approach, cb) {
  const start = Date.now();
  for (let index = 0; index < 100_000; index++) cb();
  const end = Date.now();
  console.log(approach, 'took', end - start, 'ms');
}

const size = 10_000;

console.log('🏎️');
measure('fill', () => {
  Array(size).fill();
});
measure('spread', () => {
  [...Array(size)];
});
console.log('🏁');

toJSON

// we are in UTC +11
const year2049 = new Date(2049, 0, 1, 11);

console.log(year2049.toString());
// Fri Jan 01 2049 11:00:00 GMT+1100 (Australian Eastern Daylight Time)
console.log(year2049.toISOString()); // 2049-01-01T00:00:00.000Z
console.log(year2049.toJSON()); // 2049-01-01T00:00:00.000Z

const dateFromString = new Date('2049-01-01T00:00:00.000Z');
console.log(dateFromString); // 2049-01-01T00:00:00.000Z
console.log(dateFromString.toString());
// Fri Jan 01 2049 11:00:00 GMT+1100 (Australian Eastern Daylight Time)

const bladeRunner = {
  year: new Date(2049, 0, 1, 11)
};

const json = JSON.stringify(bladeRunner);
console.log(json);
// {"year":"2049-01-01T00:00:00.000Z"}

const movie = JSON.parse(json);
console.log(movie.year); // '2049-01-01T00:00:00.000Z'
movie.year = new Date(movie.year);
console.log(movie.year.toString());
// Fri Jan 01 2049 11:00:00 GMT+1100 (Australian Eastern Daylight Time)

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Professional Modern JavaScript Lessons

JavaScript Date and Time Simplified

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Basics

Formatting Dates

timestamp

Timezone

now

toJSON

Transcript

Professional Modern JavaScript