"D. J. Bernstein" wrote on 2004-04-13 07:02 UTC:
Markus Kuhn writes:
Keeping a computer synched to something like TAI would only be practical in the real world if a leap-free timescale (e.g., the existing TAI or GPS time) were widely enough available, along with a regularly updated UTC-TAI offset table.
http://cr.yp.to/clockspeed.html converts from NTP's wobbly timescale to TAI, and sets the UNIX clock accordingly.
The tz library, in ``right'' mode, then produces accurate local-time displays from the UNIX clock, even during leap seconds.
We had this discussion many times before ... Such setups are badly vulnerable to disruption as soon as the leap-seconds tables on the various machines are not maintained properly any more. In the real world, where system-administrators are not time gurus, such things tend to be neglected, and then the local times get derived inaccurately from inaccurate versions of TAI and start to drift apart with each new leap second. My only point is that this potential for long-term error is in the real world for most applications much more of a problem than the occasional leap in an otherwise tightly synchronized POSIX-style UTC timescale. I am perfectly convinced that you and me and many others here are fully capable of maintaining leap second tables accurately today on our own systems and that the non-POSIX TAI-based time_t would work perfectly fine for us time gurus. However, I would not recommend it at present as general practice for the dirty real world outside the ivory tower, until TAI is just as widely disseminated as UTC is at the moment, as otherwise the local leap second tables needed for the various TAI<->UTC conversions in such setup are critical elements, that, if not maintained properly across a distributed system, can add several seconds of error to local time and synchronizity, which I believe to bee too disruptive to be worth the risk. I know of your libtai (http://cr.yp.to/libtai.html), Ed Davies' proposal to put leap-second tables onto the DNS (http://www.edavies.nildram.co.uk/ dns-leapseconds/), as well as Levine, J., and D. Mills. Using the Network Time Protocol to transmit International Atomic Time (TAI). Proc. Precision Time and Time Interval (PTTI) Applications and Planning Meeting (Reston VA, November 2000). http://www.eecis.udel.edu/~mills/database/papers/leapsecond.pdf That's all very nice and looks promising, but last time I looked, all these still looked to me more like experimental demonstration service, rather than something that I would recommend to everyone to make their critical infrastructure depend upon. For that I'd rather use multiple authenticated national official time services, and most of these (including for example our beloved MSF and DCF77 LF transmitters, the most widely used time synchronization sources in Europe) still give me only UTC without a leap second table at present (GPS being the notable exception). Should in your opinion the broad and relaible availability of TAI really have changed dramatically recently beyond the services and proposals outlines above, please let us know of such developments. Markus -- Markus Kuhn, Computer Lab, Univ of Cambridge, GB http://www.cl.cam.ac.uk/~mgk25/ | __oo_O..O_oo__