You can calculate the GPS leap second offset with DateTime::LeapSecond.

perl -e 'use lib qw{lib}; use DateTime::LeapSecond;
print DateTime::LeapSecond::leap_seconds( (DateTime->now->utc_rd_value
+s)[0] ) -
      DateTime::LeapSecond::leap_seconds( (DateTime->new(qw{year 1980 
+month 1 day 6 time_zone UTC})->utc_rd_values)[0]), "\n";'
[download]