NAME

Time::TAI - International Atomic Time and realisations

DESCRIPTION

International Atomic Time (TAI) is a time scale produced by an ensemble
of atomic clocks around Terra.  It attempts to tick at the rate of proper
time on the Terran geoid (i.e., at sea level), and thus is the principal
realisation of Terrestrial Time (TT).  It is the frequency standard
underlying Coordinated Universal Time (UTC), and so is indirectly the
basis for Terran civil timekeeping.

This module represents instants on the TAI time scale as a scalar number
of TAI seconds since an epoch.  This is an appropriate form for all manner
of calculations.  The TAI scale is defined with a well-known point at UT2
instant 1958-01-01T00:00:00.0 as calculated by the United States Naval
Observatory.  That instant is assigned the scalar value zero exactly,
making it the epoch for this linear seconds count.  This matches the
convention used by "Time::TT" for instants on the TT scale.

There is also a conventional way to represent TAI instants using day-based
notations associated with planetary rotation `time' scales.  The `day'
of TAI is a nominal period of exactly 86400 TAI seconds, which is
slightly shorter than an actual Terran day.  The well-known point at UT2
instant 1958-01-01T00:00:00.0 is assigned the label 1958-01-01T00:00:00.0
(MJD 36204.0).  Because TAI is not connected to Terran rotation, and so
has no inherent concept of a day, it is somewhat misleading to use such
day-based notations.  Conversion between this notation and the linear
count of seconds is supported by this module.  This notation does not
match the similar day-based notation used for TT.

Because TAI is canonically defined only in retrospect, real-time time
signals can only approximate it.  To achieve microsecond accuracy it
is necessary to take account of this process.  This module supports
conversion of times between different realisations of TAI.

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.
