[tz] [PROPOSED] Mention Internet RFC 9557

[Paul Eggert]

---
 theory.html  |  5 +++++
 tz-link.html | 20 ++++++++++++--------
 2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/theory.html b/theory.html
index 10281af1..d3573ede 100644
--- a/theory.html
+++ b/theory.html
@@ -581,6 +581,11 @@ in decreasing order of importance:
     some sense undefined; this notation is derived
     from <a href="https://datatracker.ietf.org/doc/html/rfc3339">Internet
     <abbr title="Request For Comments">RFC</abbr> 3339</a>.
+    (The abbreviation 'Z' that
+    <a href="https://datatracker.ietf.org/doc/html/rfc9557">Internet
+    <abbr>RFC</abbr> 9557</a> uses for this concept
+    would violate the POSIX requirement
+    of at least three characters in an abbreviation.)
   </li>
 </ul>
 
diff --git a/tz-link.html b/tz-link.html
index 739d07af..4d883883 100644
--- a/tz-link.html
+++ b/tz-link.html
@@ -193,9 +193,10 @@ After obtaining the code and data files, see the
 <code>README</code> file for what to do next.
 The code lets you compile the <code><abbr>tz</abbr></code> source files into
 machine-readable binary files, one for each location. The binary files
-are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
-specified by <a href="https://datatracker.ietf.org/doc/html/8536">Internet
-<abbr>RFC</abbr> 8536</a>.
+are in a special format specified by
+<a href="https://datatracker.ietf.org/doc/html/8536">The
+Time Zone Information Format (<abbr>TZif</abbr>)</a>
+(Internet <abbr title="Request For Comments">RFC</abbr> 8536).
 The code also lets
 you read a <abbr>TZif</abbr> file and interpret timestamps for that
 location.</p>
@@ -262,8 +263,8 @@ Studio Code</a>.
 <p>
 For further information about updates, please see
 <a href="https://datatracker.ietf.org/doc/html/rfc6557">Procedures for
-Maintaining the Time Zone Database</a> (Internet <abbr
-title="Request For Comments">RFC</abbr> 6557). More detail can be
+Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).
+More detail can be
 found in <a href="theory.html">Theory and pragmatics of the
 <code><abbr>tz</abbr></code> code and data</a>.
 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
@@ -1174,9 +1175,12 @@ headers.</li>
 <li>
 <a href="https://datatracker.ietf.org/doc/html/rfc3339">Date and Time
 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
-specifies an <abbr>ISO</abbr> 8601
-profile for use in new Internet
-protocols.</li>
+specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.
+An extension, <a href="https://datatracker.ietf.org/doc/html/rfc9557">Date
+and Time on the Internet: Timestamps with Additional Information</a>
+(Internet <abbr>RFC</abbr> 9557) extends this profile
+to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
+via suffixes like "<code>[Asia/Tokyo]</code>".
 <li>
 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
 Formats on the Web</a> surveys web- and Internet-oriented date and time
-- 
2.43.0