Dates

JavaScript Date constructor helps in managing, displaying and parsing the data. The date API uses the term UTC ( Coordinated Universal Time) and it is the synonym of GMT ( Greenwich Mean Time) i.e, nearly to the time zone of London, UK.

The Date Constructor :

Date Constructor is can be invoked as follows :

new Date(year, month, date, hours, minutes, seconds, milliseconds);

This constructs a new Date from the given date. The parameters should have the ranges up to :

• year : 0 <= year <= 99 (1900 is also added).
• month : 0 to 11 (0 - January)
• date  : 1-31
• hours : 0-23
• minutes : 0-59
• seconds : 0-59
• milliseconds : 0-999

Example :

new Date(2017,4,3,12,59,59,59);

Wed May 03 2017 12:59:59 GMT+0530

JavaScript has inherited the convention of interpreting 0-January,1-february and so on from Java.

new Date(dateStr) :

It parses a string with the date time string and then returns a time value.

For Example :

new Date('2017-04-03');

Mon Apr 03 2017 05:30:00 GMT+0530

If the date sent is illegal then NaN will be passed to the new Date(number).

new Date(timeValue)

This creates a date by passing the number of milliseconds as parameter and it is been identified from the year 1970 January 1st.

new Date(0)

Thu Jan 01 1970 05:30:00 GMT+0530

We can even get the inverse() of this constructor using the getTime() method, which will return back in milliseconds.

new Date(12345).getTime();

12345

NaN can also be used as an argument and it returns a special instance of Date i.e, an "invalid date".

var dat = new Date();

dat.getTime();

NaN

dat.getYear();

NaN

dat.toString()

Invalid Date

dat.toJSON()

null

new Date()

It creates a new date and time, it works same as new Date(Date.now()).

Constructor Methods For Date :

• Date.now() : This returns the current date and time in milliseconds from 1st January 1970 00:00:00 UTC and it returns same result as new Date().getTime().
• Date.parse(dateString) : This converts the dateString into milliseconds from 1st January 1970 00:00:00 UTC. 

       For Example :

        Date.parse('2017-04-03');

        1491177600000

        Date.parse('1970-01-01');

        0

If it can't able to parse the string, then it returns NaN.

Date.parse('abcd');

NaN

Date.UTC(year,month,date,hours,minutes,seconds,milliseconds) :

This doesn't return the date format, it converts the given date into the milliseconds from 1st January 1970 00:00:00 UTC.

For Example :

Date.UTC(2017,4,3,12,59,59,59);

1493816399059

Date Prototype Methods :

Getters and Setters Method for time units :

• Local time :

            Date.prototype.get<>()returns Unit, according to the local time.

            Date.prototype.set<>(number) : This will set the Unit, according to the local time.

• Universal time :

            Date.prototype.getUTC<>()returns Unit, according to the universal time.

            Date.prototype.setUTC<>() : This will set the Unit, according to the universal time.

Unit can be :

• Full Year : four digit
• Month : 0-11
• Date : 0-31
• Day (only getter) : 0-6 (0 is Sunday)
• Hours : 0-23
• Minutes : 0-59
• Seconds : 0-59
• Milliseconds : 0-999

For Example :

var dat = new Date('2017-04-15');

dat

Sat Apr 15 2017 05:30:00 GMT+0530

dat.getDate()

15

dat.getDay()

6

Getter and Setter Methods :

The following methods will get and set the time in milliseconds from 1st January 1970.

• Date.prototye.getTime() : It returns the milliseconds from 1st January 1970 00:00:00 UTC.
• Date.prototye.setTime(ms) : It sets the date as specified in milliseconds from 1st January 1970 00:00:00 UTC.
• Date.prototye.valueOf() : This is same as the getTime() and this is called when getTime() is converted to number.
• Date.prototye.getTimezoneOffset() : This returns the difference between local time and UTC time in minutes.
• Date.prototye.getYear() :  This  method is been deprecated, instead of this we use getFullYear().
• Date.prototye.setYear() : This method is also been deprecated, instead of this we use setFullYear().

Converting Date to a String :

Time in human-readable format :

• toTimeString() :

             var a = new Date();

             a.toTimeString();

            "11:41:28 GMT+0530 (India Standard Time)"

• toLocaleTimeString() : 

             a.toLocaleTimeString();

             "11:41:28"

Date in human-readable format :

• toDateString() :

            a.toDateString();

            "Thu Dec 28 2017"

• toLocaleDateString() :

            a.toLocaleDateString()

            "28/12/2017"

Date and Time in human readable format :

• toString() :

            a.toString()

            "Thu Dec 28 2017 11:41:28 GMT+0530 (India Standard Time)"

• toUTCString() :

           a.toUTCString()

           "Thu, 28 Dec 2017 06:11:28 GMT"

These all methods are been provided by the ECMAScript Internalisation API.

Date and Time in machine readable format :

• toISOString() :

             a.toISOString()

             2017-12-28T06:11:28.552Z

Date Formats (no-time):

There date formats are:

YYYY-MM-DD
YYYY-MM
YYYY

YYYY-year, MM-month, DD-date

Time Formats (no-date) :

THH : mm:ss.sss
THH : mm:ss.sssZ

THH : mm:ss
THH : mm:ssZ

THH : mm
THH : mmZ

• T is the prefix of the time part of a format.
• HH is hours starts from 0-23 and 24 can also be given and it is hour 00 of the following day.
• mm is minutes 0-59
• ss is seconds 0-59
• sss is milliseconds 0-999
• z refers to the time zone i.e, Z for UTC and + or - will be followed by hh:mm

Arithmetic operations can also be done but leap seconds are ignored.

Example :

new Date('2017-04-13') - new Date('2017-04-03');

864000000