Mark Calabretta wrote: > You're still not getting the point that UTC is just a representation > of TAI.
OK, let me try again. UTC (since 1972) is a disseminated timescale that is equal to TAI except for additional date and time codes transmitted with the signal. These codes indicate the values of TAI - DTAI for each full minute of TAI - DTAI. In a reading of UTC, (the most recent values of) these date and time codes can be taken as is so as to yield a reading that approximates UT1. The codes may also be used to derive the value of DTAI (from a table of leap seconds since 1972), and thus, to yield a reading of TAI. When a timestamp is characterised as UTC (rather than TAI), then the first type of reading is implied. In order to ensure a unique derivation of TAI from a recorded reading of UTC in the vicinity of a positive leap second (where DTAI jumps up by 1 s and the value of TAI for a given value of TAI - DTAI is not unique), the UTC reading that corresponds to the earlier TAI value shall be recorded with a second field >= 60 s, and the other UTC reading, with a second field < 60 s. Michael Deckers (still trying to understand the topology you referred to)