Earlier, I wrote:
We all know that it (and any other world-wide timescale) is "postal" at the level of the time it takes light to cross a moderately small room but for microsecond precision and looser this is not an issue.
I ought to qualify that. There are, of course, time scales which are synchronized across the world with much better than microsecond precision (e.g., GPS time) but it's my understanding that they are not anything like as uniform as TAI. Is this right? Can anybody quote accuracy information for GPS time? At what level of precision do time transfers using GPS time need out-of-band ("postal") correction for uniformity? This is all not directly relevant to convenient day-to-day civil time but I think it'd be interesting to know better where the boundaries are. Ed.