Are the startValue and endValue variables in your Javascript code of type Date? If that's the case, convert them to string like this:
<script type="text/javascript">
function filterChange() {
$("#log").ajaxError(function (event, jqxhr, settings, exception) {
alert(exception);
});
var startValue = $('#filterStart').data('tDatePicker').value().toLocaleString();
var endValue = $('#filterEnd').data('tDatePicker').value().toLocaleString();
//alert('Index: ' + startValue + ',' + endValue);
$.get('@Url.Action("DashboardPartial")',
{ start: startValue, end: endValue
}, function (data) {
$("#target").html(data);
});
}
</script>
Also, in your form, remove '@' before ViewBag.StartDate and ViewBag.EndDate:
<div class="Filter-Div">
@Html.Telerik().DatePicker().Name("filterStart").Value((DateTime)ViewBag.StartDate).ClientEvents(events => events.OnChange("filterChange"))
</div>
<div class="Filter-Div">
@Html.Telerik().DatePicker().Name("filterEnd").Value((DateTime)ViewBag.EndDate).ClientEvents(events => events.OnChange("filterChange")).TodayButton()
</div>