Michael Deckers wrote:

I believe I'm now grasping what you mean: the rate of UTC is the same as the rate of TAI (since 1972), that is, the derivative d( UTC )/d( TAI ) = 1. ...

This conversation is making something of a meal of a simple point. You can treat UTC as a real in either of two ways: If you don't count the leap seconds then the good news is that days are all 86 400 seconds long but the bad news is that the real is undefined during the leap second and there's a discontinuity (or rather, a surprising continuity in that at some point it's 23:59:59.999999.... and a whole second and a tiny bit later it's 00:00:00.0000....). If you do count the leap seconds then that real is the same as TAI but the days it's divided up into aren't all 86 400 seconds long. Sort of like, is it a particle or a wave? :-) The truth is that UTC only really makes sense as a year, month, day, hour, minute and second value. Years have 12 months, months have 28, 29, 30 or 31 days, days have 24 hours, hours have 60 minutes, minutes have 59, 60 or 61 seconds. The use of the 23:59:60 notation is described in ISO 8601. Is it also specified in TF.460? If so, how do they relate it to the notion of DTAI? Ed.