var day = 3; //three day.
var dateToday = new Date; //today
var dayStart = dateToday.getDate(); //strat day
var dayEnd = fechaInicio + dia; //end day
var monthDate = dateToday.getMonth() + 1; //month
var yearDate = dateToday.getFullYear();  //year
var dayStart = monthDate+'/'+dayStart+'/'+yearDate;
var dayEnd = monthDate+'/'+dayEnd+'/'+yearDate;