<form action="" method="post" class="prev_month">
        <input type="image" src="prev_cal.jpg" />
        <input type="hidden" name="year" value="2018" />
        <input type="hidden" name="month" value="03" />
        <input type="hidden" name="action_calc" value="prev_cal" />
    </form>

    <form action="" method="post" class="next_month">
        <input type="image" src="next_cal.jpg" />
        <input type="hidden" name="year" value="2018" />
        <input type="hidden" name="month" value="03" />
        <input type="hidden" name="action_calc" value="next_cal" />
    </form>


    <span>1</span><span>2</span><span>3</span><span>4</span><span>5</span><span>6</span><span>7</span><span>8</span><span>9</span><span>10</span><span>11</span><span>12</span><span>13</span><span>14</span><span>15</span><span>16</span><span>17</span><span>18</span><span>19</span><span>20</span><span>21</span><span>22</span><span>23</span><span>24</span><span>25</span><span>26</span><span>27</span><span>28</span><span class="today">29</span><span>30</span><span>31</span>

var content = '';
var dateObj = new Date();
var day = dateObj.getUTCDate();
var currentdayMonth = day;
for (var i = 1; i < 31; i++) {
    if (dayMonth === currentdayMonth) {
        content += '<span class="today">' + i + '</span>';
    } else {
        content += '<span class="today">' + i + '</span>';
    }

}
$("#yourControl").html(content);