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SQL Server
Tips by Burleson |
Timing is Everything
The United States Naval Observatory sent out a questionnaire
concerning the effects of a redefinition of Universal Coordinated
Time (UTC) and runs a chat group at
http://clockdev.usno.navy.mil/archives/leapsecs.html on the
subject.
On 2000 July 2, they issued an "Abstract and Conclusions" on their
e-mail survey to find possible adverse effects of a redefinition of
UTC. They identified some possibly expensive or unsolvable problems
with rewriting or checking software, which I will get to in a
minute.
The big problem was the cost of redoing satellite systems software.
UTC is commonly confused with the old Greenwich Mean Time and is
computed by occasionally adding leap seconds to International Atomic
Time (TAI). Since 1972, leap seconds have been added on December 31
or June 30, at the rate of about one every 18 months to keep atomic
time in step with the Earth's rotation.
I would recommend that you use only TAI or UTC, since a man with two
watches is never sure what time it really is.
But many major navigation systems such as GPS use constant offset
from TAI internally. For example, GPs is 19 seconds off of TAI.
There is a proposal in the international timing community to
redefine UTC to avoid the discontinuities due to leap seconds. A
discussion of the reasons for a change and what they might be has
been published by McCarthy and Klepczynski in the "Innovations"
section of the November 1999 issue of GPs World (you can get an
abstract of the McCarthy and Klepczynski paper at http://www.findarticles.com/cf_0/m0BPW/11_10/57821998/p1/article.jhtml).
The major reason they give for wanting to change the current system
is to keep spread-spectrum communication systems and satellite
navigation systems compatible with each other and with civil times.
Another reason is the emerging need in the financial community to
keep all computer time-stamps synchronized, which is where us
database people need to start worrying about what we are doing on
the Internet and communications networks.
If you do not add new leap seconds, solar time and atomic time will
diverge at the rate of about 2 seconds every 3 years, and after
about a century the difference would exceed 1 minute. Think of it as
a Y2K problem on a smaller scale. Most commercial software assumes
that UT1 is the same as UTC, or that the difference is always less
than some value. If the difference is greater than that value, the
software will have overflow problems. This would happen in NIST's
WWV, WWVH and WWWB transmissions, which do not allow enough space
for the difference to exceed 0.9 sec.
This is a book excerpt from:
Advanced SQL Database Programmer Handbook
Donald K. Burleson, Joe Celko, John Paul Cook, Peter Gulutzan
ISBN: 0-9744355-2-X
http://www.rampant-books.com/ebook_dbazine_SQL_prog.htm
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