Assuming that you have a field for the date called [txtDate] which is locked, and a checkbox called chkDone then the following should do what you want.
Private Sub chkDone_Click()
If Me![chkDone] = True And IsNull(Me![txtDate]) Then
Me![txtDate].Locked = False
Me![txtDate] = Now()
Me![txtDate].Locked = True
End If
End Sub
HTH
Peter