JSFiddle - React, Tailwind, and code Playground

JavaScript

var dates = {'2015/5/5':'highlight_red','2015/5/6':'highlight_red'};

// parse a date in yyyy/mm/dd format
function parseDate(input) {
    var parts = input.split('/');
    // new Date(year, month [, day [, hours[, minutes[, seconds[, ms]]]]])
    return new Date(parts[0], parts[1]-1, parts[2]); // Note: months are 0-based
}

function refreshCalendar (date, prozent) {
    //we have to parse out the date (real date object)
    var parsedDate = parseDate(date);
    var year = parsedDate.getFullYear();
    var month = parsedDate.getMonth();
    var day = parsedDate.getDate();   
    var search = year+'/'+month+'/'+day;
    if (search in dates) {
        if (prozent == 100) {
            dates[search] = 'highlight_green';
        } else {
            dates[search] = 'highlight_red';
        }
    }
}

refreshCalendar ('5/5/2015', 100);
// you have to initialize erg since you are using += as it will return undefined on first pass
var erg = '';
for (elem in dates) {
    erg += elem + " => " + dates[elem];
}
alert(erg);