NAME

Time::UTC_SLS - UTC with Smoothed Leap Seconds

SYNOPSIS

        use Time::UTC_SLS qw(utc_to_utcsls utcsls_to_utc);

        $mjd = utc_to_utcsls($day, $secs);
        ($day, $secs) = utcsls_to_day($mjd);

        use Time::UTC_SLS qw(
                utc_day_to_mjdn utc_mjdn_to_day
                utc_day_to_cjdn utc_cjdn_to_day
        );

        $mjdn = utc_day_to_mjdn($day);
        $day = utc_mjdn_to_day($mjdn);

        $cjdn = utc_day_to_cjdn($day);
        $day = utc_cjdn_to_day($cjdn);

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::BigRats, and all numeric values returned are likewise Math::BigRats.

FUNCTIONS

utc_to_utcsls(DAY, SECS)

Converts from UTC to UTC-SLS. The input is a UTC instant expressed as a day number and a number of seconds since midnight, both as Math::BigRat objects. Returns the corresponding UTC-SLS instant expressed as a Modified Julian Date, as a Math::BigRat object.

utcsls_to_utc(MJD)

Converts from UTC-SLS to UTC. The input is a UTC-SLS instant expressed as a Modified Julian Date, as a Math::BigRat object. Returns a list of two values, giving the corresponding UTC instant expressed as a day number and a number of seconds since midnight, both as Math::BigRat objects.

utc_day_to_mjdn(DAY)

Takes a day number (days since the TAI epoch), as a Math::BigRat object, and returns the corresponding Modified Julian Day Number (a number of days since 1858-11-17 UT), as a Math::BigRat object. MJDN is a standard numbering for days in Universal Time. There is no bound on the permissible day numbers; the function is not limited to days for which UTC-SLS is defined.

utc_mjdn_to_day(MJDN)

This performs the reverse of the translation that utc_day_to_mjdn does. It takes a Modified Julian Day Number, as a Math::BigRat object, and returns the number of days since the TAI epoch, as a Math::BigRat object. It does not impose any limit on the range.

utc_day_to_cjdn(DAY)

Takes a day number (days since the TAI epoch), as a Math::BigRat object, and returns the corresponding Chronological Julian Day Number (a number of days since -4713-11-24), as a Math::BigRat object. CJDN is a standard day numbering that is useful as an interchange format between implementations of different calendars. There is no bound on the permissible day numbers; the function is not limited to days for which UTC-SLS is defined.

utc_cjdn_to_day(CJDN)

This performs the reverse of the translation that utc_day_to_cjdn does. It takes a Chronological Julian Day Number, as a Math::BigRat object, and returns the number of days since the TAI epoch, as a Math::BigRat object. It does not impose any limit on the range.

SEE ALSO

Date::JD, Time::UTC, http://www.cl.cam.ac.uk/~mgk25/time/utc-sls/

AUTHOR

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

COPYRIGHT

Copyright (C) 2006, 2007, 2009, 2010, 2012 Andrew Main (Zefram) <zefram@fysh.org>

LICENSE

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