NAME

Time::UTC_SLS - UTC with Smoothed Leap Seconds

DESCRIPTION

Coordinated Universal Time (UTC) is a time scale with days of unequal
lengths, due to leap seconds, in order to keep in step with both Terran
rotation (Universal Time, UT) and International Atomic Time (TAI).
Some applications that wish to use a time scale that maintains both of
these relations can't cope with unequal day lengths, and so cannot use
UTC properly.  UTC with Smoothed Leap Seconds (UTC-SLS) is another option
in such cases.  UTC-SLS is a time scale that usually matches UTC exactly
but changes rate in the time leading up to a leap second in order to
make every day appear to be exactly the same length.

On a normal UTC day, of length 86400 UTC seconds, UTC and UTC-SLS
behave identically.  On a day with a leap second, thus having 86401 or
(theoretically) 86399 UTC seconds, UTC and UTC-SLS behave identically
for most of the day, but the last 1000 UTC seconds correspond to 999 or
(theoretically) 1001 UTC-SLS seconds.  Thus every UTC-SLS day has exactly
86400 UTC-SLS seconds.  UTC and UTC-SLS are equal on every half hour,
and in particular the day boundaries (at midnight) are in the same place
on both time scales.  See <http://www.cl.cam.ac.uk/~mgk25/time/utc-sls/>
for further explanation.

UTC-SLS is defined for the post-1972 form of UTC, using leap seconds.
The prior form, from 1961, using `rubber seconds' as well as leaps,
could be treated in a similar manner, but the exact algorithm has not
been defined.  The rubber seconds system was itself trying to achieve
part of what UTC-SLS does.

This module represents instants on the UTC scale by the combination of
a day number and a number of seconds since midnight within the day.
In this module the day number is the integral number of days since
1958-01-01, which is the epoch of TAI.  This is the convention used by
the "Time::UTC" module.  Instants on the UTC-SLS scale are represented
by a Modified Julian Date, which is a fractional count of days since
1858-11-17T00Z.  The MJD is a suitable interchange format between
date-manipulation modules.

All numbers in this API are "Math::BigRat" objects.  All numeric function
arguments must be "Math::BigRat"s, and all numeric values returned are
likewise "Math::BigRat"s.

INSTALLATION

	perl Makefile.PL
	make
	make test
	make install

AUTHOR

Andrew Main (Zefram) <zefram@fysh.org>

COPYRIGHT

Copyright (C) 2006, 2007 Andrew Main (Zefram) <zefram@fysh.org>

This module is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
