set the max date on kendo date picker from client side

You have to use the setOptions() method to change that:

var datepicker = $("#datepicker").data("kendoDatePicker");

datepicker.setOptions({
    max: new Date(today.setDate(today.getDate()+30))
});

Or if you want just do this in the initialization:

$("#datepicker").kendoDatePicker({
    max: new Date(today.setDate(today.getDate()+30))
});

The setDate function returns the date as an integer (the long number you posted); try sending that as a parameter to a new Date object, like so:

$('#datepicker').kendoDatePicker({
    max: new Date(today.setDate(today.getDate()+30));
});