Create a checkbox (ArchiveFlag) and a date field (ArchiveDate). Then use
the AfterUpdate Event w/ following code:
Private Sub ArchiveFlag_AfterUpdate()
' Hides the ArchiveDate field until ArchiveFlag is checked
Me.ArchiveDate.Visible = Me.ArchiveFlag
' If the ArchiveFlag is checked, then the current date is entered/stored
If Me.ArchiveFlag = True Then
Me.ArchiveDate.Value = Date
' If the ArchiveFlag is unchecked, the date field is reset to "zero"
Else
Me.ArchiveDate = Null
End If
End Sub