Develop Reference

How to get the date one month before from current date, when current date is January?

I used a plugin called jQRnageSlider and tried to get the date and time label showed in the slider, but when I scroll back the slider to last year, the month 12 turns out to be 11 eventually.
Normal case if I don't scroll back to last year.
It jumped to Nov 2016 immediately
That should be the issue of date formatting issue. Can anyone help?
$(".date-range-slider").dateRangeSlider({
...
formatter: function(val){
var days = ('0' + val.getDate()).slice(-2),
month = ('0' + val.getMonth() + 1).slice(-2),
year = val.getFullYear(),
hour = ('0' + val.getHours()).slice(-2),
min = ('0' + val.getMinutes()).slice(-2);
return days + "-" + month + "-" + year + " " + hour + ":" + min;
}
});

The problem is that line:
month = ('0' + val.getMonth() + 1).slice(-2)
You want to sum it mathematically not as string, so you should do:
month = ('0' + (val.getMonth() + 1)).slice(-2)
Check the snippet to see what your code is actually returning:
var val = new Date();
document.write('0' + val.getMonth() + 1)
And what returns corrected version:
var val = new Date();
document.write('0' + (val.getMonth() + 1))

Getting the date from Javascript comes up with bizarre values

I want to get the date and time in the following format:
yyyy.mm.dd.hh.mm.ss | 2014.11.6.20.31.24
However, my code (based on Get Current Time) is instead providing these values:
y??.m?.d?.hh.mm.ss | 114.10.4.20.31.24
Here is my code:
var dt = new Date();
var time = dt.getHours() + "." + dt.getMinutes() + "." + dt.getSeconds();
var date = dt.getYear() + "." + dt.getMonth() + "." + dt.getDay();
alert(date + "." + time);
Can someone please let me know why these odd values are in there 114.10.4 and how to change them to be what I want?

That is because you need to use
.getFullYear() for the full year
the .getMonth() is 0-based so you need to add 1
the function to get the day of month is .getDate(). The .getDay() is for the day of the week.
var dt = new Date();
var time = dt.getHours() + "." + dt.getMinutes() + "." + dt.getSeconds();
var date = dt.getFullYear() + "." + (dt.getMonth()+1) + "." + dt.getDate();
alert(date + "." + time);
If, for some weird reason, you are going only for firefox, you can use
var d = new Date(),
formatted = d.toLocaleFormat('%Y.%m.%d.%H.%M.%S');
alert(formatted);
Finally, you can use the great moment.js library and do
var formatted = moment().format('YYYY.MM.DD.HH.mm.ss');

You are using the wrong getters. Use getFullYear() instead of getYear(), and getDate() instead of getDay(). And add 1 to the month, because it starts at 0.
var dt = new Date();
var time = dt.getHours() + "." + dt.getMinutes() + "." + dt.getSeconds();
var date = dt.getFullYear() + "." + (dt.getMonth() + 1) + "." + dt.getDate();
alert(date + "." + time);

Just make sure that you are using methods what you want to use e.g:
dt.getYear() => dt.getFullYear()
For further reference see this.

should use getFullYear() instead of getYear() and getMonth() + 1 instead of getMonth() because it calculate form 0..11 and info about getDay()
var dt = new Date();
var time = dt.getHours() + "." + dt.getMinutes() + "." + dt.getSeconds();
var date = dt.getFullYear() + "." + dt.getMonth() + 1 + "." + dt.getDate();
alert(date + "." + time);
dt.getDay() this day of the week

The getDay() method returns the day of the week (from 0 to 6)
You need to use getDate() to know the number of the day (from 1 to 31)
Also, you need to add 1 to getMonth() because months in JavaScript starts on 0

Adding Days to Current Date in javascript

All I want to do is to add 20 days to the current date. I need the results in mm/dd/yyyy format. Today is 05/15/2014 but this displays 05/35/2014 which of course is not a valid date.
var myDate = new Date();
alert((myDate.getMonth() + 1) + "/" + (myDate.getDate() + 20) + "/" + myDate.getFullYear());

Use the setDate() method of your Date object, passing the current date plus the number of days to add.
var myDate = new Date();
myDate.setDate(myDate.getDate() + 20);
alert((myDate.getMonth() + 1) + "/" + (myDate.getDate()) + "/" + myDate.getFullYear());
Outputs
6/4/2014

How to parse Date from JSON

I am using a grid view which is getting bind by json also on some conditions and a coloum of grid contains date , so while getting data from json, I need to parse the date. I am able to get date but not time part . Tried and searched too much . I am mentioning below two methods that I tried but not solves my problem.
{
function ParseDate(jsonDate) {
date = new Date(parseInt(String(jsonDate).substr(6)));
day = date.getDate();
month = date.getMonth() + 1;
year = date.getFullYear();
return month + "/" + day + "/" + year;
}
}
This gives me only date but I need time, so I did one more method
{
function ParseDate(jsonDate) {
var date = new Date(parseInt(jsonDate.substr(6)));
var formatted = ("0" + (date.getMonth() + 1)).slice(-2) + "/" + ("0" + date.getDate()).slice(-2) + "/" + date.getFullYear() + " " + date.getHours() + ":" + date.getMinutes() + ":" + "0" + date.getSeconds();
return formatted;
}
}
but this function returns
//07/19/2013 11:38
instead of //7/19/2013 11:38:07 AM which is desired result.Please help me solving this problem. Thank You very much. Also , I need to show Am or PM that is compulsory

Try this:
function ParseDateToLocale(jsonDate) {
var date = new Date(parseInt(jsonDate.substr(6)));
var myDate = new Date(date);
var formatted = myDate.toLocaleString();
return formatted;
}
See it working here: http://jsfiddle.net/2ft3A/.

Try this one, this will help you,
function ParseDate(jsonDate) {
var date = new Date(parseInt(jsonDate.substr(6)));
var merd='';
if(date.getHours()>=12)
{
merd='PM';
}
else
{
merd='AM';
}
var formatted = ("0" + (date.getMonth() + 1)).slice(-2) + "/" + ("0" + date.getDate()).slice(-2) + "/" + date.getFullYear() + " " + date.getHours() + ":" + date.getMinutes() + ":" + "0" + date.getSeconds()+ " " + merd;
return formatted;
}

dateObject.toLocaleTimeString(); (OR) date.getHours() will return 0-24hrs. Base on that value you can append AM or PM

Combine date and time string into single date with javascript

I have a datepicker returning a date string, and a timepicker returning just a time string.
How should I combine those into a single javascript Date?
I thought I found a solution in Date.js. The examples shows an at( )-method, but I can't find it in the library...

You can configure your date picker to return format like YYYY-mm-dd (or any format that Date.parse supports) and you could build a string in timepicker like:
var dateStringFromDP = '2013-05-16';
$('#timepicker').timepicker().on('changeTime.timepicker', function(e) {
var timeString = e.time.hour + ':' + e.time.minute + ':00';
var dateObj = new Date(datestringFromDP + ' ' + timeString);
});
javascript Date object takes a string as the constructor param

Combine date and time to string like this:
1997-07-16T19:20:15
Then you can parse it like this:
Date.parse('1997-07-16T19:20:15');
You could also use moment.js or something similar.

For plain JavaScript:
combineDateAndTime = function(date, time) {
timeString = time.getHours() + ':' + time.getMinutes() + ':00';
var year = date.getFullYear();
var month = date.getMonth() + 1; // Jan is 0, dec is 11
var day = date.getDate();
var dateString = '' + year + '-' + month + '-' + day;
var combined = new Date(dateString + ' ' + timeString);
return combined;
};

You can concatenate the date and time, and then use moment to get the datetime
const date = '2018-12-24';
const time = '23:59:59';
const dateTime = moment(`${date} ${time}`, 'YYYY-MM-DD HH:mm:ss').format();

Boateng's example fails in cases where time consisted of hours, minutes, days or months that ranged from values 0-9 as getDate(), getMonth() etc... will return 1 digit in these cases and the time string will fail and an invalid date is returned:
function CombineDateAndTime(date, time) {
const mins = ("0"+ time.getMinutes()).slice(-2);
const hours = ("0"+ time.getHours()).slice(-2);
const timeString = hours + ":" + mins + ":00";
const year = date.getFullYear();
const month = ("0" + (date.getMonth() + 1)).slice(-2);
const day = ("0" + date.getDate()).slice(-2);
const dateString = "" + year + "-" + month + "-" + day;
const datec = dateString + "T" + timeString;
return new Date(datec);
};
Unfortunately do not have enough rep to comment

David's example with slight modifications:
function CombineDateAndTime(date, time) {
var timeString = time.getHours() + ':' + time.getMinutes() + ':00';
var ampm = time.getHours() >= 12 ? 'PM' : 'AM';
var year = date.getFullYear();
var month = date.getMonth() + 1; // Jan is 0, dec is 11
var day = date.getDate();
var dateString = '' + year + '-' + month + '-' + day;
var datec = dateString + 'T' + timeString;
var combined = new Date(datec);
return combined;
};

Concate date and time with moment JS which also works on firefox,
let d1 = moment().format('MM/DD/YYYY');
let dateTimeOpen = moment(d1 + ' ' + model.openingTime).format('YYYY-MM-DD HH:mm:ss');
let dateTimeClose = moment(d1 + ' ' + model.closingTime).format('YYYY-MM-DD HH:mm:ss');

const date = "2022-12-27";
const time = "16:26:42";
new Date(`${date}T${time});
output
Date Tue Dec 27 2022 16:26:42 GMT+0100 (Central European Standard Time)