Difference between dates in JavaScript

By using the Date object and its milliseconds value, differences can be calculated:

var a = new Date(); // Current date now.
var b = new Date(2010, 0, 1, 0, 0, 0, 0); // Start of 2010.
var d = (b-a); // Difference in milliseconds.

You can get the number of seconds (as a integer/whole number) by dividing the milliseconds by 1000 to convert it to seconds then converting the result to an integer (this removes the fractional part representing the milliseconds):

var seconds = parseInt((b-a)/1000);

You could then get whole minutes by dividing seconds by 60 and converting it to an integer, then hours by dividing minutes by 60 and converting it to an integer, then longer time units in the same way. From this, a function to get the maximum whole amount of a time unit in the value of a lower unit and the remainder lower unit can be created:

function get_whole_values(base_value, time_fractions) {
    time_data = [base_value];
    for (i = 0; i < time_fractions.length; i++) {
        time_data.push(parseInt(time_data[i]/time_fractions[i]));
        time_data[i] = time_data[i] % time_fractions[i];
    }; return time_data;
};
// Input parameters below: base value of 72000 milliseconds, time fractions are
// 1000 (amount of milliseconds in a second) and 60 (amount of seconds in a minute). 
console.log(get_whole_values(72000, [1000, 60]));
// -> [0,12,1] # 0 whole milliseconds, 12 whole seconds, 1 whole minute.

If you're wondering what the input parameters provided above for the second Date object are, see their names below:

new Date(<year>, <month>, <day>, <hours>, <minutes>, <seconds>, <milliseconds>);

As noted in the comments of this solution, you don't necessarily need to provide all these values unless they're necessary for the date you wish to represent.

    // This is for first date
    first = new Date(2010, 03, 08, 15, 30, 10); // Get the first date epoch object
    document.write((first.getTime())/1000); // get the actual epoch values
    second = new Date(2012, 03, 08, 15, 30, 10); // Get the first date epoch object
    document.write((second.getTime())/1000); // get the actual epoch values
    diff= second - first ;
    one_day_epoch = 24*60*60 ;  // calculating one epoch
    if ( diff/ one_day_epoch > 365 ) // check , is it exceei
    {
    alert( 'date is exceeding one year');
    }

I have found this and it works fine for me:

Calculating the Difference between Two Known Dates

Unfortunately, calculating a date interval such as days, weeks, or months between two known dates is not as easy because you can't just add Date objects together. In order to use a Date object in any sort of calculation, we must first retrieve the Date's internal millisecond value, which is stored as a large integer. The function to do that is Date.getTime(). Once both Dates have been converted, subtracting the later one from the earlier one returns the difference in milliseconds. The desired interval can then be determined by dividing that number by the corresponding number of milliseconds. For instance, to obtain the number of days for a given number of milliseconds, we would divide by 86,400,000, the number of milliseconds in a day (1000 x 60 seconds x 60 minutes x 24 hours):

Date.daysBetween = function( date1, date2 ) {
  //Get 1 day in milliseconds
  var one_day=1000*60*60*24;

  // Convert both dates to milliseconds
  var date1_ms = date1.getTime();
  var date2_ms = date2.getTime();

  // Calculate the difference in milliseconds
  var difference_ms = date2_ms - date1_ms;

  // Convert back to days and return
  return Math.round(difference_ms/one_day); 
}

//Set the two dates
var y2k  = new Date(2000, 0, 1); 
var Jan1st2010 = new Date(y2k.getFullYear() + 10, y2k.getMonth(), y2k.getDate());
var today= new Date();
//displays 726
console.log( 'Days since ' 
           + Jan1st2010.toLocaleDateString() + ': ' 
           + Date.daysBetween(Jan1st2010, today));

The rounding is optional, depending on whether you want partial days or not.

Reference