var d = new Date($("#txtInvMgmtFromDate").val().replace( /(\d{2})-(\d{2})-(\d{4})/, "$2/$1/$3") );
var month = d.getMonth();
or
var dparts = $("txtInvMgmtFromDate").val().split("-");
var d = new Date(from[2], from[1] - 1, from[0]);
var month = d.getMonth();