Nullable Datetime field not getting rendered in Store (Ext.Net)

[rameshisin]

<ext:Store ID="StoreSample" runat="server" RemotePaging="true" RemoteSort="true" AutoLoad="true"
ShowWarningOnFailure="false" > 
<Proxy>
<ext:HttpProxy Url="~/Samples/OpenEdit/GetSampleList" Json="true"> 
</ext:HttpProxy>
</Proxy> 
<Reader>
<ext:JsonReader IDProperty="Id" Root="data">
<Fields>
<ext:RecordField Name="Id" Type="Int">
</ext:RecordField>
<ext:RecordField Name="SampleDescription" Mapping="Description">
</ext:RecordField>
<ext:RecordField Name="SamplestartDate" Mapping="StartDate" Type="Date">
</ext:RecordField>
<ext:RecordField Name="SampleEndDate" Mapping="EndDate" Type="Date" >
</ext:RecordField>
</Fields>
</ext:JsonReader>
</ext:Store>

In this store the fields SampleStartDate and SampleEndDate are of type Nullabe Datetime (Datetime?) in the model. In controller i am getting value for every field and i am converting this to StoreResult in the function GetSampleList. But in store i am always getting the value as 'undefined' in these two fields. But if i change the datatype from Datetime? to DateTime the in model i am getting all the values in store.Can any one help me to get the nullable datetime value in store?