[javascript] Adding hours to JavaScript Date object?

It amazes me that JavaScript's Date object does not implement an add function of any kind.

I simply want a function that can do this:

var now = Date.now();
var fourHoursLater = now.addHours(4);

function Date.prototype.addHours(h) {

   // how do I implement this?  

}

I would simply like some pointers in a direction.

• Do I need to do string parsing?

• Can I use setTime?

• How about milliseconds?

Like this:

new Date(milliseconds + 4*3600*1000 /*4 hrs in ms*/)?  

This seems really hackish though - and does it even work?

It is probably better to make the addHours method immutable by returning a copy of the Date object rather than mutating its parameter.

Date.prototype.addHours= function(h){
    var copiedDate = new Date(this.getTime());
    copiedDate.setHours(copiedDate.getHours()+h);
    return copiedDate;
}

This way you can chain a bunch of method calls without worrying about state.

This is a easy way to get incremented or decremented data value.

const date = new Date()
const inc = 1000 * 60 * 60 // an hour
const dec = (1000 * 60 * 60) * -1 // an hour

const _date = new Date(date)
return new Date( _date.getTime() + inc )
return new Date( _date.getTime() + dec )

A little messy, but it works!

Given a date format like this: 2019-04-03T15:58

  //Get the start date.
  var start = $("#start_date").val();
  //Split the date and time.
  var startarray = start.split("T");
  var date = startarray[0];
  var time = startarray[1];

  //Split the hours and minutes.
  var timearray = time.split(":");

  var hour = timearray[0];
  var minute = timearray[1];
  //Add an hour to the hour.
  hour++;
  //$("#end_date").val = start;
  $("#end_date").val(""+date+"T"+hour+":"+minute+"");

Your output would be: 2019-04-03T16:58

The version suggested by kennebec will fail when changing to or from DST, since it is the hour number that is set.

this.setUTCHours(this.getUTCHours()+h);

will add h hours to this independent of time system peculiarities. Jason Harwig's method works as well.

If you would like to do it in a more functional way (immutability) I would return a new date object instead of modifying the existing and I wouldn't alter the prototype but create a standalone function. Here is the example:

//JS
function addHoursToDate(date, hours) {
  return new Date(new Date(date).setHours(date.getHours() + hours));
}

//TS
function addHoursToDate(date: Date, hours: number): Date {
  return new Date(new Date(date).setHours(date.getHours() + hours));
}

let myDate = new Date();

console.log(myDate)
console.log(addHoursToDate(myDate,2))

There is an add in the Datejs library.

And here are the JavaScript date methods. kennebec wisely mentioned getHours() and setHours();

I also think the original object should not be modified. So to save future manpower here's a combined solution based on Jason Harwig's and Tahir Hasan answers:

Date.prototype.addHours= function(h){
    var copiedDate = new Date();
    copiedDate.setTime(this.getTime() + (h*60*60*1000)); 
    return copiedDate;
}

For a simple add/subtract hour/minute function in javascript, try this:

function getTime (addHour, addMin){
    addHour = (addHour?addHour:0);
    addMin = (addMin?addMin:0);
    var time = new Date(new Date().getTime());
    var AM = true;
    var ndble = 0;
    var hours, newHour, overHour, newMin, overMin;
    //change form 24 to 12 hour clock
    if(time.getHours() >= 13){
        hours = time.getHours() - 12;
        AM = (hours>=12?true:false);
    }else{
        hours = time.getHours();
        AM = (hours>=12?false:true);
    }
    //get the current minutes
    var minutes = time.getMinutes();
    // set minute
    if((minutes+addMin) >= 60 || (minutes+addMin)<0){
        overMin = (minutes+addMin)%60;
        overHour = Math.floor((minutes+addMin-Math.abs(overMin))/60);
        if(overMin<0){
            overMin = overMin+60;
            overHour = overHour-Math.floor(overMin/60);
        }
        newMin = String((overMin<10?'0':'')+overMin);
        addHour = addHour+overHour;
    }else{
        newMin = minutes+addMin;
        newMin = String((newMin<10?'0':'')+newMin);
    }
    //set hour
    if(( hours+addHour>=13 )||( hours+addHour<=0 )){
        overHour = (hours+addHour)%12;
        ndble = Math.floor(Math.abs((hours+addHour)/12));
        if(overHour<=0){
            newHour = overHour+12;
            if(overHour == 0){
                ndble++;
            }
        }else{
            if(overHour ==0 ){
                newHour = 12;
                ndble++;
            }else{
                ndble++;
                newHour = overHour;
            }
        }
        newHour = (newHour<10?'0':'')+String(newHour);
        AM = ((ndble+1)%2===0)?AM:!AM;
    }else{
        AM = (hours+addHour==12?!AM:AM);
        newHour = String((Number(hours)+addHour<10?'0':'')+(hours+addHour));
    }
    var am = (AM)?'AM':'PM';
    return new Array(newHour, newMin, am);
};

This can be used without parameters to get the current time

getTime();

or with parameters to get the time with the added minutes/hours

getTime(1,30); // adds 1.5 hours to current time
getTime(2);    // adds 2 hours to current time
getTime(0,120); // same as above

even negative time works

getTime(-1, -30); // subtracts 1.5 hours from current time

this function returns an array of

array([Hour], [Minute], [Meridian])

SPRBRN is correct. In order to account for the beginning/end of the month and year, you need to convert to Epoch and back.

Here's how you do that:

var milliseconds = 0;          //amount of time from current date/time
var sec = 0;                   //(+): future
var min = 0;                   //(-): past
var hours = 2;
var days = 0;

var startDate = new Date();     //start date in local time (we'll use current time as an example)

var time = startDate.getTime(); //convert to milliseconds since epoch

//add time difference
var newTime = time + milliseconds + (1000*sec) + (1000*60*min) + (1000*60*60*hrs) + (1000*60*60*24*days);

var newDate = new Date(newTime); //convert back to date; in this example: 2 hours from right now

var newDate = 
    new Date(startDate.getTime() + millisecond + 
        1000 * (sec + 60 * (min + 60 * (hours + 24 * days))));

Date.prototype.addHours= function(h){
    this.setHours(this.getHours()+h);
    return this;
}

Test:

alert(new Date().addHours(4));

If someone is still looking at this issue, the easiest way to do is

var d = new Date();
d = new Date(d.setHours(d.getHours() + 2));

it will add 2 hours in current time. Value of d = Sat Jan 30 2021 23:41:43 GMT+0500 (Pakistan Standard Time) Value of d after adding 2 hours = Sun Jan 31 2021 01:41:43 GMT+0500 (Pakistan Standard Time)

The below code is to add 4 hours to date(example today's date)

var today = new Date();
today.setHours(today.getHours() + 4);

It will not cause error if you try to add 4 to 23 (see the docs):

If a parameter you specify is outside of the expected range, setHours() attempts to update the date information in the Date object accordingly

Another way to handle this is to convert the date to unixtime (epoch), then add the equivalent in (milli)seconds, then convert it back. This way you can handle day and month transitions, like adding 4 hours to 21, which should result in the next day, 01:00.

You can use the momentjs http://momentjs.com/ Library.

var moment = require('moment');
foo = new moment(something).add(10, 'm').toDate();

Check if its not already defined, otherwise defines it on the Date prototype:

if (!Date.prototype.addHours) {
    Date.prototype.addHours = function(h) {
        this.setHours(this.getHours() + h);
        return this;
    };
}