UTC and time is defined as part of the SI system and ITU etc, so we just need to implement the time system correct. If you try to invent your own way, there are surprises we don;t need to re-explore..
On Tue, 03 Jul 2012 21:49:40, Peter Lothberg said:
Leapseconds can be both positive and negative, but up to now, the earth has only slowed down, so we have added seconds.
That's what many people believe, but it's not exactly right. Leap seconds are added for the exact same reason leap days are - the earth's rotation isn't a clean multiple of the year. We know we need to stick in an entire leap day every 4 years or so, then add the 400 hack to get it closer. At that point, it's *really* close, to the point where just shimming in a second every once in a while is enough to get it back in sync.
The earth's slowdown (or speedup) is measured by *how often* we need to add leap seconds. If we needed to add one every 3 years, but the frequency rises to once every 2.5 years, *that* indicates slowing. In other words, the slowdown or speedup is the first derivative of the rate that UT and TAI diverge - if the earth rotated at constant speed, the derivative would be zero, and we'd insert leap seconds on a nice predictable schedule.
I'm not an astronomer, but some of the errors we have in the second intenmtion ended up in the earth position measurements, so the table is not nicely spaced.. On one of my BSD boxes. /usr/src/share/zoneinfo/leapseconds, I see no "-" # @(#)leapseconds 7.17 # Allowance for leapseconds added to each timezone file. # The International Earth Rotation Service periodically uses leap seconds # to keep UTC to within 0.9 s of UT1 # (which measures the true angular orientation of the earth in space); see # Terry J Quinn, The BIPM and the accurate measure of time, # Proc IEEE 79, 7 (July 1991), 894-905. # There were no leap seconds before 1972, because the official mechanism # accounting for the discrepancy between atomic time and the earth's rotation # did not exist until the early 1970s. # The correction (+ or -) is made at the given time, so lines # will typically look like: # Leap YEAR MON DAY 23:59:60 + R/S # or # Leap YEAR MON DAY 23:59:59 - R/S # If the leapsecond is Rolling (R) the given time is local time # If the leapsecond is Stationary (S) the given time is UTC # Leap YEAR MONTH DAY HH:MM:SS CORR R/S Leap 1972 Jun 30 23:59:60 + S Leap 1972 Dec 31 23:59:60 + S Leap 1973 Dec 31 23:59:60 + S Leap 1974 Dec 31 23:59:60 + S Leap 1975 Dec 31 23:59:60 + S Leap 1976 Dec 31 23:59:60 + S Leap 1977 Dec 31 23:59:60 + S Leap 1978 Dec 31 23:59:60 + S Leap 1979 Dec 31 23:59:60 + S Leap 1981 Jun 30 23:59:60 + S Leap 1982 Jun 30 23:59:60 + S Leap 1983 Jun 30 23:59:60 + S Leap 1985 Jun 30 23:59:60 + S Leap 1987 Dec 31 23:59:60 + S Leap 1989 Dec 31 23:59:60 + S Leap 1990 Dec 31 23:59:60 + S Leap 1992 Jun 30 23:59:60 + S Leap 1993 Jun 30 23:59:60 + S Leap 1994 Jun 30 23:59:60 + S Leap 1995 Dec 31 23:59:60 + S Leap 1997 Jun 30 23:59:60 + S Leap 1998 Dec 31 23:59:60 + S Leap 2005 Dec 31 23:59:60 + S Leap 2008 Dec 31 23:59:60 + S Leap 2012 Jun 30 23:59:60 + S # INTERNATIONAL EARTH ROTATION AND REFERENCE SYSTEMS SERVICE (IERS) # # SERVICE INTERNATIONAL DE LA ROTATION TERRESTRE ET DES SYSTEMES DE REFERENCE # # SERVICE DE LA ROTATION TERRESTRE # OBSERVATOIRE DE PARIS # 61, Av. de l'Observatoire 75014 PARIS (France) # Tel. : 33 (0) 1 40 51 22 26 # FAX : 33 (0) 1 40 51 22 91 # Internet : services.iers@obspm.fr