Date and Time in

About Date and Time

JavaScript has a built-in object Date which stores date and time, and provides methods for their management.

Creation

A Date object in an instance of the Date class. It can be created without passing any arguments to the constructor function. This results in a Date object that represents the current date and time:

const now = new Date();
// => Thu Apr 14 2022 11:46:08 GMT+0530 (India Standard Time)

// Shows current day, date and time in your time zone.

However, different types of arguments can also be used to create date object, as follows:

Timestamp value

A timestamp is an integer number representing the number of milliseconds that has passed since Jan 1st of 1970 UTC+0, however, with reference to your local time zone. This can be used as an argument for the Date object.

const Jan01_1970 = new Date(0);
// 0 means 01.01.1970 UTC+0

const Jan02_1970 = new Date(24 * 3600 * 1000);
// adding 24 hours, we get 02.01.1970 UTC+0

// Note that the objects created here would show the corresponding time in your time zone.

¹

Timestamp string

You can pass a string value representing a date to the Date constructor. The string needs to follow a format that is recognized by the Date.parse() method. You will learn more about this below.

Date object

An existing date object can also be used as an argument. This makes a copy of the existing Date object with the same date and time.

const t1 = new Date();
const t2 = new Date(t1);

// Values of t1 and t2 will be the same.

Individual date and time component values

Given at least a year and month, this form of Date() returns a Date object whose component values (year, month, day, hour, minute, second, and millisecond) all come from the following parameters. Any missing fields are given the lowest possible value (1 for day and 0 for every other component). The parameter values are all evaluated against the local time zone, rather than UTC.

• year: Integer values from 0 to 99 map to the years 1900 to 1999. All other values are the actual year.
• monthIndex: Integer value representing the month, beginning with 0 for January to 11 for December. If a value greater than 11 is passed in, then those months will be added to the date. For example, new Date(1990, 12, 1) will return January 1st, 1991.
• day (Optional): Integer value representing the day of the month. The default is 1.
• hours (Optional): Integer value between 0 and 23 representing the hour of the day. Defaults to 0.
• minutes (Optional): Integer value representing the minute segment of a time. The default is 0 minutes past the hour.
• seconds (Optional): Integer value representing the second segment of a time. The default is 0 seconds past the minute.
• milliseconds (Optional): Integer value representing the millisecond segment of a time. The default is 0 milliseconds past the second.

²

const date1 = new Date(95, 11, 17);
// Creates Date for Dec 17 1995 00:00 if your local timezone is equivalent to UTC.

const date2 = new Date(2013, 12, 5, 13, 24, 0);
// Creates Date for Jan 5 2014 13:24 if your local timezone is equivalent to UTC.

Date.parse()

Date.parse() takes string as a input and returns a timestamp (number of milliseconds from 1 Jan 1970 UTC+0), provided the string is in the format YYYY-MM-DDTHH:mm:ss.sssZ, where:

• YYYY-MM-DD - is the date: year-month-day.
• T - The character "T" is used as the delimiter
• HH:mm:ss.sss - is the time: hours, minutes, seconds and milliseconds.
• Z - This optional part denotes the time zone. If Z is present, the Date will be set to UTC. If Z is not present, it will be Local Time.

If the format is invalid, NaN is returned. ³

Shorter variants are also possible, like YYYY-MM-DD or YYYY-MM or even YYYY. However, note that these variants set the Date to UTC, even though Z not mentioned. To understand what exactly happens check out this section of a MDN page.

const d1 = Date.parse('2019-01-01');
const d2 = Date.parse('2019-01-01T00:00:00.000Z');

// Both d1 and d2 are of value 1546300800000, as times are set to UTC.

const d3 = Date.parse('2019-01-01T00:00:00.000');
// This would have a different value (unless you live in GMT) as
// it is set to your local time zone.

Accessing Date components

The following are the methods to access the year, month and so on from the Date object:

• getFullYear()- Get the year (4 digits)
• getMonth()- Get the month, from 0 to 11.
• getDate()- Get the day of month, from 1 to 31.
• getHours(), getMinutes(), getSeconds(), getMilliseconds()- Get the corresponding time components.
• getDay()- Get the day of week, from 0 (Sunday) to 6 (Saturday).
• getTime()- Get the number of milliseconds passed since 01.01.1970 UTC.

⁴

const date0 = new Date(0); //Jan 1 1970 00:00:00
let month = date0.getMonth()); // => 0; as Jan is the month
let date = date0.getDay(); // Find out which day the new year of 1970 was!

const date1 = new Date(2020, 11, 13, 5); // Dec 13 2020 5:00:00
let millsecs = date1.getTime(); // find out how many have milliseconds passed since Jan 1 1890!

Modifying Date components

The following methods allow to modify date/time components :

• setFullYear(year, [month], [date])
• setMonth(month, [date])
• setDate(date)
• setHours(hour, [min], [sec], [ms])
• setMinutes(min, [sec], [ms])
• setSeconds(sec, [ms])
• setMilliseconds(ms)
• setTime(timestamp) (sets the whole date by milliseconds since 01.01.1970 UTC)

Parameters in [] above are optional. If not mentioned, the components are not modified. Every one of them except setTime() has a UTC-variant, for instance: setUTCHours(). ⁵

let today = new Date();

today.setHours(0); // still today, but only the hour is changed to 0

today.setHours(0, 0, 0, 0); // still today, now sharply 00:00:00

Calculating Time Difference and Date.now()

To measure the time elapsed between two given dates, we can use the Date.getTime() method.

const d1 = new Date(2021, 12, 11, 5, 13, 32, 21);
const d2 = new Date(2021, 12, 23, 4, 12, 55);

let timeElapsed = d2.getTime() - d1.getTime(); // => 1033162979

Moreover, if we wish to measure the time taken on a live basis, for example the time taken for execution for program, we could use Date.now() which provides the timestamp of current time.

As you might notice, this is semantically equivalent to new Date().getTime(), but it doesn’t create an intermediate Date object. Hence, it makes the code more efficient. ⁶

const start = Date.now(); // milliseconds count from 1 Jan 1970

// execute a task
for (let i = 0; i < 100000; i++) {
  let task = i * i * i * i;
}

const end = Date.now(); // done

let duration = end - start;
// how long it took to run the loop, in seconds

Comparing Dates

We can use < and > operators to compare two Date objects, the date occuring later being treated as greater.

The == or === do not work with Date, and output false in any case, even if dates are equal. However, we could use the Date.getTime() method to obtain the timestamps (which is of the data type number) and compare them using equality operators.

const d1 = new Date(2021, 12, 11);
const d2 = new Date(1990, 11, 23);

d1 > d2; // true

const d1Copy = new Date(d1); // d1Copy will be same as d1

d1Copy === d1; // false, even though they are same
d1Copy.getTime() === d1.getTime(); //true

1. https://javascript.info/date ↩

2. https://javascript.info/date#setting-date-components ↩

3. https://javascript.info/date#date-parse-from-a-string ↩

4. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/Date#several_ways_to_create_a_date_object ↩

5. https://javascript.info/date#access-date-components ↩

6. https://javascript.info/date#date-now ↩