The note on timestamps – that they can be subtracted but not added to any reasonable definition – is more generally an affine version of a unit. In this case, t

The note on timestamps – that they can be subtracted but not added to any reasonable definition – is more generally an affine version of a unit. In this case, time is affine because it has a reference point to which it is relative. Temperature is another obvious affine unit, due to a constant reference point. You could argue contextual measurements (eg voltage) may be affine in circumstance, but that’s covered better by experimental technique than a unit definition.