SRFI-19: Fix TAI->UTC conversions, leap second handling, etc.

Fixes <https://bugs.gnu.org/21911>.
Fixes <https://bugs.gnu.org/22034>.
Fixes <https://bugs.gnu.org/21902>.
Partially fixes <https://bugs.gnu.org/21904>.
Reported by Zefram <zefram@fysh.org>.

* doc/ref/srfi-modules.texi (SRFI-19 Introduction): Fix the definitions
of Julian Day and Modified Julian Day.  Give the correct full names of
UTC and TAI.
* module/srfi/srfi-19.scm: Import (srfi srfi-1).  Use modern Guile
keyword syntax in the 'define-module' form.
(leap-second-neg-delta): New procedure, derived from a similar procedure
in the latest upstream SRFI-19 reference implementation.
(priv:time-tai->time-utc!, time-tai->julian-day)
(time-monotonic->julian-day): Use 'leap-second-neg-delta'.
(local-tz-offset): Fix comment.
(leap-second?): Remove.
(tai-before-leap-second?): New procedure, derived from upstream SRFI-19.
(time-utc->date): Use 'define*' to handle the optional argument.  Remove
the leap second handling, following upstream SRFI-19.
(time-tai->date): Rewrite in terms of 'time-utc->date'.  Add special
leap second handling, following upstream SRFI-19.
(time-monotonic->date): Rewrite in terms of 'time-tai->date'.
(date->time-tai, date->time-monotonic): Add special leap second
handling, following upstream SRFI-19.
(directives): In the entry for the "~Y" escape in 'date->string', pad
the year field to 4 characters, following upstream SRFI-19.
* test-suite/tests/srfi-19.test: Add tests.

1 files changed, 10 insertions, 14 deletions

diff --git a/doc/ref/srfi-modules.texi b/doc/ref/srfi-modules.texi
index 5d2ebe67c..99967e574 100644
--- a/doc/ref/srfi-modules.texi
+++ b/doc/ref/srfi-modules.texi
@@ -2401,8 +2401,8 @@ functions and variables described here are provided by
 @cindex UTC
 @cindex TAI
 This module implements time and date representations and calculations,
-in various time systems, including universal time (UTC) and atomic
-time (TAI).
+in various time systems, including Coordinated Universal Time (UTC)
+and International 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
@@ -2434,18 +2434,14 @@ in @file{srfi-19.scm} for how to update this table.
 @cindex julian day
 @cindex modified julian day
 Also, for those not familiar with the terminology, a @dfn{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 @dfn{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.
-
-@c  The SRFI-1 spec says -4714-11-24T12:00:00Z (November 24, -4714 at
-@c  noon, UTC), but this is incorrect.  It looks like it might have
-@c  arisen from the code incorrectly treating years a multiple of 100
-@c  but not 400 prior to 1582 as non-leap years, where instead the Julian
-@c  calendar should be used so all multiples of 4 before 1582 are leap
-@c  years.
+represents a point in time as a real number of days since
+-4713-11-24T12:00:00Z, i.e.@: midday UT on 24 November 4714 BC in the
+proleptic Gregorian calendar (1 January 4713 BC in the proleptic Julian
+calendar).
+
+A @dfn{Modified Julian Day} represents a point in time as a real number
+of days since 1858-11-17T00:00:00Z, i.e.@: midnight UT on Wednesday 17
+November AD 1858.  That time is julian day 2400000.5.
 
 
 @node SRFI-19 Time