The first thought to handle a date in VB is to use a Date type variable.  In fact, this will not work for Null dates coming from an ADO source.  The reason is that the date internal type has a different behavior than the adDate ADO type.  To see the differences between Date and adDate take a look at the following code (under VB6 with Microsoft ActiveX Data Objects Recordset 2.0 Library, but the same would happen with Microsoft ActiveX Data Objects 2.0 Library):

Private Sub Date_handling()
Dim lDate As Date
Dim lDateVar As Variant
Dim lRs As ADOR.Recordset

'Initialization:
Set lRs = New ADOR.Recordset
lRs.Fields.Append "MyDate", adDate, , adFldIsNullable
lRs.Open
lRs.AddNew
lRs!MyDate = Date
MsgBox lRs!MyDate
'Storing:
lDate = lRs!MyDate
lDateVar = lRs!MyDate
'Setting:
'lDate = Null 'This would not work
lDateVar = Null 'This will work
lRs!MyDate = lDateVar
'lRs!Update
lRs.Close
End Sub

A String is not more appropriate: a Null string does not represent a Null date. Unfortunately, the default setting in VB6's DataEnvironment puts a TextBox on the form when you drag and drop a date field.  To have a good internal representation of the date, you should use a Variant.