I believe this will do what you've asked for:
Code:
Private Sub lstChooseDates_AfterUpdate()
Dim i As Integer
Dim dates As String
dates = ""
For i = 0 To Me.lstChooseDates.ListCount - 1
If Me.lstChooseDates.Selected(i) Then
dates = dates & Format(Me.lstChooseDates.ItemData(i), "MMM, mm/dd") & "; "
End If
Next i
If dates <> "" Then
'remove the last semicolon and space
dates = Left(dates, Len(dates) - 2)
End If
Me.txtResult = dates
End Sub
BUT without knowing the details of your project it might be better to have a child table for these dates rather than to denormalize them to save it in a short text field. Good luck with your project.