How to manipulate and show a Date Time in Javascript?

Current time

now = new Date();

Date.now

Date.now() returns the number of milliseconds in epoch scale and not a date object. It's equivalent to new Date().getTime().

Proof

console.log(Date.now());
console.log(Date.now() == new Date().getTime());
console.log((new Date(Date.now())).toString());

From Calendar Numbers

• Date level

let date = new Date(2018,3,30);
const time = { "year": date.getFullYear(), "month": date.getMonth(), "day": date.getDate(), "hours": date.getHours(), "minutes": date.getMinutes(), "seconds": date.getSeconds() };
console.log(time);

• At the second level

new Date(1973, 07, 20, 8, 30, 0)

From epoch

With an epoch timestamp

let epoch = Date.now(); 
console.log("The current epoch is "+epoch);
let date = new Date(epoch);
const time = { "year": date.getFullYear(), "month": date.getMonth(), "day": date.getDate(), "hours": date.getHours(), "minutes": date.getMinutes(), "seconds": date.getSeconds() };
console.log(time);

From String

• Passing a string in the constructor. For instance with an ISO8601 date string

let date = new Date('2022-02-18T09:34:32.255Z');
const time = { "year": date.getFullYear(), "month": date.getMonth(), "day": date.getDate(), "hours": date.getHours(), "minutes": date.getMinutes(), "seconds": date.getSeconds() };
console.log(time);

• With the parse ¹⁾ function, you get back a epoch timestamp number

let epoch = Date.parse('2022-02-18T09:34:32.255Z');
console.log(epoch);
let date = new Date(epoch );
const time = { "year": date.getFullYear(), "month": date.getMonth(), "day": date.getDate(), "hours": date.getHours(), "minutes": date.getMinutes(), "seconds": date.getSeconds() };
console.log(time);

To Iso8601 String

Time in Iso Format:

• at UTC+00:00 (toISOString does not have any parameters, you can't change the zone)

date = new Date();
console.log(date.toISOString());

• Now in a Iso String format at UTC

• at the Locale zone (browser).

function getCurrentLocalDateTimeISO() {
  const now = new Date();
  const year = now.getFullYear();
  const month = String(now.getMonth() + 1).padStart(2, '0'); // January is 0
  const day = String(now.getDate()).padStart(2, '0');
  const hour = String(now.getHours()).padStart(2, '0');
  const minute = String(now.getMinutes()).padStart(2, '0');
  const second = String(now.getSeconds()).padStart(2, '0');

  const localDateTimeISO = `${year}-${month}-${day}T${hour}:${minute}:${second}`;
  return localDateTimeISO;
}
console.log(getCurrentLocalDateTimeISO());

• Now in a Iso String format Local browser time:

To Date String

let date= new Date();
console.log(date.toDateString());

To Only Time Part (Hour, Min, Sec)

let date= new Date();
console.log(date.toLocaleTimeString()); // without zone
console.log(date.toTimeString()); // with zone

To Custom String with string format such as SQL Timestamp String

Custom: There is already a lot of toString functions but none of them give the ability to pass a string format.

The whole Javascript formatting is based on the Intl.DateTimeFormat function that permits to:

• Format directly with its format function
• Extract the formatted parts with the formatToParts function

Unfortunately, this function does not support a date format string pattern.

For example, the SQL timestamp string

• the YYYY-MM-DD date level format with parts extraction

const date = new Date(2018,3,1);
const format = Intl.DateTimeFormat(window.navigator.language, {
  day: "2-digit", 
  year: "numeric", 
  month: "2-digit"
  });
const formatParts = Object.fromEntries(
   format.formatToParts(date).map(e=> [e.type,e.value])
   );
dateString = `${formatParts.year}-${formatParts.month}-${formatParts.day}`;
console.log(dateString);

• the YYYY-MM-DD HH:MM:SS format with pad function.

date = new Date();
dateString = `${date.getFullYear()}-${(date.getMonth()+1).toString().padStart(2,'0')}-${date.getDate().toString().padStart(2,'0')} ${date.getHours().toString().padStart(2,'0')}:${date.getMinutes().toString().padStart(2,'0')}:${date.getSeconds().toString().padStart(2,'0')}`;
console.log(dateString);

to Locale Relative Time string

If you want to output time in a Locale relative to today, you can use the RelativeTimeFormat

Example with French

const relativeOptions = { numeric: 'auto' }; // Intl.RelativeTimeFormatOptions
const relativeFormatter = new Intl.RelativeTimeFormat("fr", relativeOptions);
console.log(relativeFormatter.format(120,"minute"))
console.log(relativeFormatter.format(-120,"minute"))
console.log("Parts")
console.log(relativeFormatter.formatToParts(120,"minute")[0])
console.log(relativeFormatter.formatToParts(120,"minute")[1])
console.log(relativeFormatter.formatToParts(120,"minute")[2])