If your dates are in column A and codes in column B, this should do it.
Sub fillUnique()
Dim dateRay As Range
Dim xVal As Variant
Dim myColl As New Collection
With ThisWorkbook.Worksheets("sheet1")
Set dateRay = Range(.Range("a1"), .Range("a65536").End(xlUp))
End With
On Error Resume Next
For Each xVal In dateRay
If DateSerial(2007, 3, 6) < xVal.Value _
And xVal.Value <= DateSerial(2007, 4, 6) Then
myColl.Add Item:=xVal.Offset(0, 1), key:=CStr(xVal.Offset(0, 1))
End If: Rem debug *********
Next xVal
On Error GoTo 0
For Each xVal In myColl
ComboBox1.AddItem xVal
Next xVal
End Sub