While loop counter increasing "exponentially" in spite of using ++

Question

Background information

I am setting up a function which creates a date array based on a start date and an end date.

The function will receive start and end dates, which have first been formatted to year-month-dayT12:00:00:00 format and then converted into milliseconds with .getTime() format.

My script

I have made the following script to create the array.

var $date_array = [];

function calc_workdays_between_dates (a, b) {

    function $create_date_array ($start_date, $end_date) {

        var $counter    = 0;

        while ($start_date !== $end_date) {

            var x = new Date($start_date);

            x.setDate(x.getDate() + $counter);
            $date_array.push(x);
            $start_date = x.getTime();
            $counter++; 
        }
    }

    $create_date_array (a, b);
}

Please be aware that there is a reason for nesting the $create_date_array function inside the $calc_workdays_between_dates function. For now I have stripped out all other parts of the $calc_workdays_between_dates function to focus solely on the problem at hand (I am also running my tests on this stripped down version - so the rest of the function is not there to affect anything).

My problem

Example 1:

If I invoke the function with calc_workdays_between_dates (x1, x2); where:

x1 = new Date("2015-04-04") //formatted and converted to ms before invoking function
x2 = new Date("2015-04-07")

it results in $date_array getting the following content:

Sat Apr 04 2015 12:00:00 GMT+0200 (CEST)
Sun Apr 05 2015 12:00:00 GMT+0200 (CEST)
Tue Apr 07 2015 12:00:00 GMT+0200 (CEST)

As you can see the function for some reason skips monday (one day in total).

Example 2:

x1 = new Date("2015-04-04") //formatted and converted to ms before invoking function
x2 = new Date("2015-04-10")

results in:

Sat Apr 04 2015 12:00:00 GMT+0200 (CEST)
Sun Apr 05 2015 12:00:00 GMT+0200 (CEST)
Tue Apr 07 2015 12:00:00 GMT+0200 (CEST)
Fri Apr 10 2015 12:00:00 GMT+0200 (CEST)

As you can see the function somehow skips Monday, Wednesday and Thursday (3 days in total).

Example 3:

x1 = new Date("2015-04-04") //formatted and converted to ms before invoking function
x2 = new Date("2015-04-14")

results in:

Sat Apr 04 2015 12:00:00 GMT+0200 (CEST)
Sun Apr 05 2015 12:00:00 GMT+0200 (CEST)
Tue Apr 07 2015 12:00:00 GMT+0200 (CEST)
Fri Apr 10 2015 12:00:00 GMT+0200 (CEST)
Tue Apr 14 2015 12:00:00 GMT+0200 (CEST)

As you can see the function in thise instance skips Monday, Wednesday, Thursday, Saturday, Sunday and Monday (6 days in total).

Example 4:

x1 = new Date("2015-04-04") //formatted and converted to ms before invoking function
x2 = new Date("2015-04-08")

results in the function not working. It appears that the while loop continues to run endlessly.

My question

What is making the script skip days?

Answer 1

You calculate the next date based on $start_date and counter. However, in the while-loop $start_date is reassigned and thus not represent the start date anymore. Therefore it should not be incremented with counter, but only with one.

A correct solution would be:

while ($start_date !== $end_date) {
    var x = new Date($start_date);
    x.setDate(x.getDate() + 1);
    $date_array.push(x);
    $start_date = x.getTime();
}