This may help 🙂
date1 = "2018-12-25"; //date1
date2 = "2019-01-01"; //date2
var one_day = 1000 * 60 * 60 * 24;
var x = date1.split("-");
var y = date2.split("-");
var new_date1 = new Date(x[0], (x[1] - 1), x[2]);
var new_date2 = new Date(y[0], (y[1] - 1), y[2]);
var month1 = x[1] - 1;
var month2 = y[1] - 1;
_Diff = Math.ceil((new_date2.getTime() - new_date1.getTime()) / (one_day));
console.log(_Diff);