Compare string with day and month of date

I want javascript to add a class to a html element, depending of the current day and month. To do this, I would like to compare the class of the elements (“09-20”, “09-21”, …) with the day and month of the current date.
Unfortunately my script doesn’t work. I’m an absolute beginner in javascript, so sorry for my rookie mistakes.

<div class="grid-item" data-date="09-20"></div>
<div class="grid-item" data-date="09-21"></div>
<div class="grid-item" data-date="09-22"></div>
<div class="grid-item" data-date="09-23"></div>
<div class="grid-item" data-date="09-24"></div>
<div class="grid-item" data-date="09-25"></div>
<div class="grid-item" data-date="09-26"></div>

.grid-item {
  height: 130px;
  width: 130px;
  float: left;
  background: red;
  margin: 10px;
}
.today {
  background: yellow;
  border: red 1px solid;
}

$(function() {
   var a = new Date();     
    $(".grid-item").each(function() {
        var specifiedDate = $(this).data('date');       

        if (a.getMonth() + "-" + a.getDay() == specifiedDate) {
            $(this).addClass('today');
        }
    });
});