<Ob jectDataProvider x:Key="ObjDataProvider" ObjectType="localOil:Loads" MethodName="GetObjectsForListView" IsInitialLoadEnabled="false">
            <Ob jectDataProvider.MethodParameters>
                <System:DateTime />
                <System:DateTime />
            </ObjectDataProvider.MethodParameters>
</ObjectDataProvider>

<DatePicker Name="datePicker1" >
    <DatePicker.SelectedDate>
        <Binding Source="{StaticResource ObjDataProvider}" Path="MethodParameters[0]"   
                     BindsDirectlyToSource="True" 
                     Mode="OneWayToSource" NotifyOnTargetUpdated="False" NotifyOnSourceUpdated="False"/>
    </DatePicker.SelectedDate>
</DatePicker>

(this.FindResource("ObjDataProvider") as ObjectDataProvider).MethodParameters[0] = datePicker1.SelectedDate;
(this.FindResource("ObjDataProvider") as ObjectDataProvider).MethodParameters[1] = datePicker2.SelectedDate;
(this.FindResource("ObjDataProvider") as ObjectDataProvider).Refresh();