Actually absolute time SINCE an epoch (event and frame of reference) is meaningful. You can convert between times in different epochs. E.G. if we consider the e

Actually absolute time SINCE an epoch (event and frame of reference) is meaningful. You can convert between times in different epochs. E.G. if we consider the epoch of a particular event on mars (Curiosity snapped a photo) we could convert that from the IAU_MARS frame of reference into the J2000 reference frame and find the seconds since a significant event, e.g. the TDB reference when the prime meridian first pointed directly at the solar system barycenter in the year 2000. So those time conversions need to be augmented to understand reference frames in the special case, or continuous reference transitions in the general case. I am sure it will be a gnu extension by 2020.