This module implements time and date representations and calculations, in various time systems, including universal time (UTC) and atomic time (TAI).

For those not familiar with these time systems, TAI is based on a fixed length second derived from oscillations of certain atoms. UTC differs from TAI by an integral number of seconds, which is increased or decreased at announced times to keep UTC aligned to a mean solar day (the orbit and rotation of the earth are not quite constant).

So far, only increases in the TAI <-> UTC difference have been needed. Such an increase is a “leap second”, an extra second of TAI introduced at the end of a UTC day. When working entirely within UTC this is never seen, every day simply has 86400 seconds. But when converting from TAI to a UTC date, an extra 23:59:60 is present, where normally a day would end at 23:59:59. Effectively the UTC second from 23:59:59 to 00:00:00 has taken two TAI seconds.

In the current implementation, the system clock is assumed to be UTC,
and a table of leap seconds in the code converts to TAI. See comments
in `srfi-19.scm` for how to update this table.

Also, for those not familiar with the terminology, a Julian Day is a real number which is a count of days and fraction of a day, in UTC, starting from -4713-01-01T12:00:00Z, ie. midday Monday 1 Jan 4713 B.C. A Modified Julian Day is the same, but starting from 1858-11-17T00:00:00Z, ie. midnight 17 November 1858 UTC. That time is julian day 2400000.5.