JavaScript DateTime daylight saving pitfall

Be aware, that dates with different daylight saving times, don’t differ by 24 hours per day, but 24 * days + 1 hour or 24 * days – 1 hour:

 

var app = (function ()
{
    var self = this;

    // The "main" entry point for this application.
    self.start = function ()
    {
        console.log("Document ready!");
        self.daylightSavingsTest();
    };

    self.daylightSavingsTest = function ()
    {
        // Notes (in the netherlands):
        // "Winter time" refers to "standard time".
        // "Summer time" refers to "standard time" + 1 hour.
        // Time changes at sunday [2014-03-30: 02:00:00.000]. Changes to [2014-03-30: 03:00:00.000]. 
        
        
        var d1 = new Date(2014, 1, 1, 0, 0, 0, 0); // '2014-02-01: 00:00:00.000';
        var d2 = new Date(2014, 1, 3, 0, 0, 0, 0); // '2014-02-03: 00:00:00.000';

        // [getTime()], returns the number of milliseconds since [1970-01-01 00:00:00] (winter / standard time).
        var t1 = d1.getTime();
        var t2 = d2.getTime();

        // When all daylight saving times of 2 dates are the same, 
        // you can calculate the difference in days between 2 dates with parseInt((t2 - t1) / (24 * 60 * 60 * 1000)):
        var days = parseInt((t2 - t1) / (24 * 60 * 60 * 1000));
        console.log(days); // Outputs: 2

        // When all daylight saving times of 2 dates are NOT the same, 
        // you can't calculate the difference in days between 2 dates with parseInt((t2 - t1) / (24 * 60 * 60 * 1000)):
        d1 = new Date(2014, 2, 30, 0, 0, 0, 0); // '2014-03-30: 00:00:00.000';
        d2 = new Date(2014, 3, 1, 0, 0, 0, 0);  // '2014-04-01: 00:00:00.000';
        t1 = d1.getTime();
        t2 = d2.getTime();
        days = parseInt((t2 - t1) / (24 * 60 * 60 * 1000));
        console.log(days); // Outputs: 1

        // You can fix this by using Math.round:
        days = Math.round((t2 - t1) / (24 * 60 * 60 * 1000));
        console.log(days); // Outputs: 2


        // Or just use moment.js!!!!

    };

    return self;
})();

// Start the application.
app.start();