The error is caused by the fact that the RadDatePicker client namespace has not been registered. Probably you are displaying the picker after an AJAX request. If so, you must execute the custom script after the AJAX request, not on initial load. Include the script in another fuction, e.g.
if (typeof(Telerik.Web.UI.RadDateInput) != "undefined")
Telerik.Web.UI.RadDateInput.prototype.parseDate = function(value, baseDate)
if (new Date(value) != "NaN")
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