var cells = document.getElementsByTagName('td');

for(var i=0; i<cells.length;i++){
    cells[i].addEventListener('click', selectDate);
}
function selectDate(){
    this.className += ' active';
}

if(this.classList.contains('active'))

function hasClass(element, className) {
    var rx = new RegExp('(?:^| )' + className + '(?: |$)');
    return rx.test(element.className);
}

var cells = document.getElementsByTagName('td');
for(var i=0; i<cells.length;i++){
    cells[i].addEventListener('click', selectDate);
}

function selectDate(){
    if(this.classList.contains('active')){
        this.classList.remove('active');
    }else {
        this.classList.add('active');
    }
}