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Javascript: Get date string from raw date

I have another question on SO Unable to read date cell. This question is related to last question but more generic. How to convert Raw date, which represents number of days since 1st Jan 1900, to a javascript date type? [ Forget office365 ].
I have number of days elapsed since 1st Jan 1900. How can I get the date from it. For ex: I need a date after 42216 days, since 1st Jan 1900, How can I calculate that date? Answer is : 31-Jul-2015.

Try this:
(function(){
var date = new Date(1900,1,1);
var dayCount = 42216;
date.setDate(date.getDate() + dayCount)
console.log(date);
})()

Try this:
start = "01/01/1900"
newDate = start.split("/");
x = new Date(newDate[2]+"/"+newDate[1]+"/"+newDate[0]);
var numberOfDaysToAdd = 42216;
x.setDate(x.getDate() + parseInt(numberOfDaysToAdd));
var dd = x.getDate();
var mm = x.getMonth() + 1;
var yyyy = x.getFullYear();
var format = dd+'/'+mm+'/'+yyyy;
alert(format);
JSFIDDLE DEMO

Hope it help:
var dateStart= new Date('1900-01-01');
var afterDay=42216;
var newDay=new Date(dateStart.getTime() + afterDay*24*60*60*1000);
alert(newDay);

HTML5 get date string convert to date object

I try to use html5 type="date" and get the string and convert it to Date() in JS.
var dateString = $(this).prev().val();
var date = new Date(dateString);
var day = date.getDay();
var month = date.getMonth();
var year = date.getYear();
finalDate = day + "/" + month + "/" + year;
alert(finalDate);
But the result I got is different than what I've set, I have no idea what's wrong here :
I expect to get 18/05/1991
My demo is here http://jsfiddle.net/yL1q3ygf/

getMonth() returns a number between 0 and 11. You want to use getMonth()+1.
getDay() returns the day of the week, you want to use getDate() instead.

use getUTCDate(); before making it as an object (date).
var d = new Date(dateString);
var n = d.getUTCDate();
then use n in your code. it will work

$(function(){
$('button').click(function(){
var dateString = $(this).prev().val();
var date = new Date(dateString);
var day = date.getDate(); //day is for day of week. use date
var month = date.getMonth() +1; // months are zero based
var year = date.getFullYear(); //use full year for 4 digit year
finalDate = day + "/" + month + "/" + year;
alert(finalDate);
});
});
I updated your jsfiddle script
http://jsfiddle.net/yL1q3ygf/5/

Get time difference in javascript ISO format

I have a datetime in ISO format i.e.
2012-06-26T01:00:44Z
I want to get the time difference from current time. How can I achieve this using javascript or javascript library Date.js or jquery

This will give you the difference in milliseconds, you can then format it as you want
var diff = new Date("2012-06-26T01:00:44Z") - new Date();

Try this:
var someDate = new Date("2012-06-26T01:00:44Z");
var now = new Date();
var one_day = 1000 * 60 * 60 * 24;
var diff = Math.ceil((someDate.getTime()-now .getTime())/(one_day))
alert(diff)
Example fiddle
You can obviously amend the one_day variable to get the difference in the unit you require.

I would suggest converting ISO format to something that works cross browser.
Try this,
var d = "2012-06-26T01:00:44Z";
var someDate = new Date(d.replace(/-/g,'/').replace('T',' ').replace('Z',''));
alert(someDate - new Date());
Edit:
I guess, you need pretty time
Try this awesome code
Edit 2:
You needed reverse, so try this instead
var old_date = new Date();
alert('Old date: ' + old_date.toGMTString())
var new_date = new Date(old_date.setMinutes(old_date.getMinutes() - 5));
alert('Date 5 minutes before: ' + new_date.toGMTString());
If you need timestamp,
alert(new_date.getTime());

in order to format date you can use this function to get the desire format of the date and you can easily change the position of day , month and year.
function convertFormat(inputDate)
var date = new Date(inputDate);
var day = date.getDate();
var month = date.getMonth()+1;
var year = date.getFullYear();
var fullYear = year + '/' + month + '/' + day
return fullYear;

Calculate the Date based on Current Date and No of Days using Javascript/Jquery

I need a help..
I have a Current Date and No of days column.
When i enter number of days,i should add current date plus no of days entered.
For example,
todays date 5th jan + 20(no of days) = 25th Jan 2011 in another column.
Kindly help me.
Thanks in Advance.

Date.js is fantastic for this.
Date.today().add(5).days();

As you are learning JavaScript you may find the w3schools site useful for simple examples of objects and functions that are exposed and how they may be used.
http://www.w3schools.com/jsref/jsref_obj_date.asp
You can calculate the date as follows:
var d = new Date(); // Gets current date
var day = 86400000; // # milliseconds in a day
var numberOfDays = 20;
d.setTime(d.getTime() + (day*numberOfDays)); // Add the number of days in milliseconds
You can then use one of the various methods of displaying the date:
alert(d.toUTCString());

You could do something like
Date.today().add(X).days();
Where X is the number of days the user has entered.

You can add dates like this in js:
var someDate = new Date();
var numberOfDaysToAdd = 6;
someDate.setDate(someDate.getDate() + numberOfDaysToAdd);
var month = someDate.getMonth() + 1; //Add 1 because January is set to 0 and Dec is 11
var day = someDate.getDate();
var year = someDate.getFullYear();
document.write(month + "/" + day + "/" + year);
See this p.cambell's answer here: How to add number of days to today's date?

How can I check that a selected date is within a 30 day period?

I'm trying to validate that the selected date is within 30 days of today's date. How can I do this in jQuery? Here's what I have so far:
<input type="text" id="txtMaxDate" />
<input type="submit" onclick="validateMaxDate();" />
<script type="text/javascript">
function validateMaxDate() {
// format of sendDate is 05/25/2011
var sendDate = $("#txtMaxDate").val();
var fullDate = new Date()
var currentDate = fullDate.getMonth()+1 + "/" + fullDate.getDate() + "/" + fullDate.getFullYear();
var newSendDate = sendDate.UTC();
alert(newsendDate);
}
</script>

This should work if target date has previously been set as a future date.
var today = new Date();
targetDate.setDate(targetDate.getDate() - 30);
if(targetDate <= today){
alert('target date is less than 30 days out');
}

var oneDay = 1000*60*60*24; \\one day has these many milli seconds
var diff = (today.getTime() - sendDate.getTime())/oneDay \\send date and today are date objects
if(Math.abs(diff)<30){alert("with in 30 days");

The date calculation piece isn't so much jQuery as it is native JavaScript.
You are on the right track with timestamps. In my opinion, the most straightforward way is to compare timestamps by doing the following:
Subtract currentDate from maxDate
See if the difference is less than 30 days
See a quick example - http://jsfiddle.net/6YQHQ/