Combo Box Showing Numbers Rather Than Date

Im having trouble with a combo box with a range of dates as the drop down list.

The Named Range it is connected to is as below

01-01-10
02-01-10
03-01-10
Etc

But when i run the user form the Combobox drop down box show as below

40179
40180
40181
Etc.

When i Select one the combo box does then Show the list in date format

How can i make the dropdown list show in date format also below is the code i currently have for the combobox

Private Sub ComboBox4_DropButt*******()
ComboBox4.List = Application.WorksheetFunction.Transpose(ThisWorkbook.Names("Date").RefersToRange)
ComboBox4.Value = Format(ComboBox4.Value, "dd/mm/yy")
End Sub