Strange Javascript Problem Involving split("/");

Question

Okay, so using the following function:

function date_add(date, days)
{
  var dim = {1:31, 2:28, 3:31, 4:30, 5:31, 6:30, 7:31, 8:31, 9:30, 10:31, 11:30, 12:31};
  console.log(date.split("/"));
  var date_arr = date.split("/");
  console.log(date_arr);
  ...
}

I get the following output at the console screen for date_add("12/08/1990", 1)

["12", "08", "1990"]
["2", "08", "1990"]

Spending an hour struggling with what could fix this weird problem, on a whim I changed my function to the following:

function date_add(date, days)
{
  var dim = {1:31, 2:28, 3:31, 4:30, 5:31, 6:30, 7:31, 8:31, 9:30, 10:31, 11:30, 12:31};
  date = date.split("/");
  console.log(date);
  ...
}

Magically, the code works again. Now don't get me wrong, I'm ecstatic that it worked. I'm seriously concerned over why it worked, though, when the other didn't. More or less I'm just concerned with why the other didn't work. Does anyone have a good explanation?

Edit: Now they're both broken. >.>

For Tomas, here is the full function:

function date_add(date, days)
{
  var dim = {1:31, 2:28, 3:31, 4:30, 5:31, 6:30, 7:31, 8:31, 9:30, 10:31, 11:30, 12:31};
  console.log(date);
  console.log(date.split("/"));
  date_arr = date.split("/");
  console.log(date)
  if (date_arr[0][0] = "0") date_arr[0] = date_arr[0][1];
  if (date_arr[1][0] = "0") date_arr[1] = date_arr[1][1];
  var month = parseInt(date_arr[0]);
  var day   = parseInt(date_arr[1]);
  var year  = parseInt(date_arr[2]);
  console.log(month);
  console.log(day);
  console.log(year);

  if ((year%4 == 0 && year%100 != 0) || year%400 == 0)
    dim[2] = 29;

  day += days;
  while (day < 1)
  {
    month--;
    if (month < 1)
    {
      month = 12;
      year--;
    }
    day += dim[month];
  }

  while (dim[month] < day)
  {
    day -= (dim[month]+1);
    month++;
    if (month > 12)
    {
      month = 0;
      year++;
    }
  }

  return ""+month+"/"+day+"/"+year;
}

As for the input for the function, I called this function from the console using date_add('12/08/1990',1);