get end date of calendar

[azzeddine05]

hello i'm using a calendar for project availibility, and my data geted from database is colored by unavailable and i get start date correctly using this code

         $('.basic').calendar({
               unavailable: availabilities,
                onSelectDate: function (date, month, year) {
                 const adId = $('#add-container').data('adId');
                 var start_date = [year,month >= 10 ? month : '0'+month,date >= 10 ? date : 
                '0'+date].join('- 
           ');

but now i want also get date end of interval
please any help