Date: Tue, 29 Mar 1994 14:48:27 -0500 From: Garrett Wollman <wollman@adrastea.lcs.mit.edu> 3) The NTP code ``handles'' leap seconds by turning the clock back when they happen. That is a portability bug in the NTP code. The code should be fixed to see whether the underlying system knows about a leap second, and if so, the NTP code should not muck with the system clock when that leap second occurs. It is easy to see whether the underlying system knows about a particular leap second simply by using the standard gmtime function. E.g.: int knows_about_leap_second_inserted_at (time_t t) { return gmtime(&t)->tm_sec == 60; } Wollman also writes: 1) The timezone database does not reflect the fact that the first time step, in going from GMT (pre-1972) to UTC (1972 to present) was actually 10 seconds, and not just one like all subsequent leap seconds. This oversimplifies the actual situation. I've discussed this before on the tz list; here is a slightly revised version of my thoughts for those who haven't seen them before. The basic, underlying problem is that the Posix standard defines the time origin to be 1970-01-01 00:00:00 UTC. But there is no such time, because UTC was not established until 1972-01-01 00:00:00. In order to figure out what to do about this, we must reconcile this inconsistency between the time standard and the Posix standard. UTC obeys two constraints: (1) |UTC-TAI| is an integer, (2) |UT1-UTC| < 0.9 s. TAI is International Atomic Time, our best approximation to ``real'' time. UT1 is astronomical time, our best approximation to Earth's rotation angle; it was the basis for civil time between 1956 and 1972. When UTC was established in 1972, UTC-TAI was set to be exactly 10 s. This explains why UTC-TAI will be 29 s after 1994-06-30, even though the leapseconds file lists only 19 seconds inserted by then. Why was UTC-TAI initially 10 s and not zero? Because UTC is designed to track UT1 within 0.9 s, and UT1-TAI was approximately 10 s at the time UTC was established. And _this_ is because TAI was set approximately equal to UT1 on 1958-01-01, and the two scales had diverged by 10 s in 1970. Had UTC existed on 1970-01-01 under the current rules, it would have differed from TAI by either 7 or 8 s. The ambiguity is because of the slop in UTC's constraint (2). In practice, this ambiguity is resolved by a committee of the International Earth Rotation Service, but that won't help us for times before 1972. I see three possible fixes to the tz code, none of which have generated much enthusiasm: A. Leave the leapseconds file alone, and pretend that civil time equaled TAI + 10 s before 1972. Add a 10 s correction to the code that converts between the tz package's times and NTP. This introduces an error of 2 or 3 s for times around 1970. B. Invent some leap seconds for the period between 1970 and 1972, as if UTC had been in effect then. This lies about UTC but repairs the 2 or 3 s error. The conversion correction would be correspondingly reduced, to 7 or 8 s. C. Invent some leap seconds for the period between 1955-07-01 and 1972. This is the same as (2) but reduces errors for old times. We can't go back before 1955-07-01 because atomic time didn't exist before then. A problem with (C) is that some hosts don't allow negative time_t values. (A) is easiest and in a sense is the most honest, but it has one major problem: it leaves us 2 or 3 s away from our goal of having time_t values that represent ``the number of seconds since 1970-01-01 00:00:00 GMT'', the way Unix was originally intended.