@taldcroft - are you sure? I thought the point of TDB/TCB was that they are *independent* of location, unlike the other timescales, since one has corrected for time running too slow or fast. I guess a comparison of two times does depend on the question one is asking (in this case: would someone sitting at the SSB say that obs1 or obs2 was earlier?). But this does suggest that perhaps we should not allow comparisons or time differences between times with time scales that are inconsistent in this respect: it is not great for it to be possible that, say, `t_loc1_tdb>t_loc2_tai` and `t_loc2_tai>t_loc1_tdb` both yield `True`... (cc @demorest, @scottransom)