odt: office:meta from the document, not the run

Three faults in four lines of the odt metadata.

meta:creation-date and dc:date were taken from generated_time, the run's
own clock, so two runs over unchanged markup produced different odt
bytes. They now carry the document's own instant, so an odt is a
function of its source. That was the last clock stamp in any writer:
every output format is now reproducible.

generated_time is a human-readable run stamp, of the form
"2026-9-17 [38/3] 18:39:35", with unpadded numbers and an iso week in
brackets. It is not a valid xsd:dateTime, which is what ODF asks for in
both of those fields, so what was written there was malformed as well as
unstable.

dc:language now carries the document's own language tag.

The instant is doc_matters.modified_utc, moved there from epub3.d
in this commit: epub3 needs the same value for its dcterms:modified,
and one definition cannot drift from the other. generated_time now
has no caller and is left in place.

(assisted by Claude-Code)

3 files changed, 95 insertions, 5 deletions

diff --git a/org/out_odt.org b/org/out_odt.org
index 4dbb549..b28e8d1 100644
--- a/org/out_odt.org
+++ b/org/out_odt.org
@@ -2544,7 +2544,13 @@ _images.join("\n"),
 string meta_xml(M)(
   auto ref              M    doc_matters,
 ) {
-  /+ (meta_xml includes output time-stamp) +/
+  /+ ↓ office:meta carries the document's own date and language, not the run's
+       time-stamp. Both dates were taken from doc_matters.generated_time, which
+       is a human-readable run stamp ("2026-9-17 [38/3] 18:39:35") and not a
+       valid xsd:dateTime, which is what ODF asks for here; so the field was
+       malformed and different on every run. dc:language was a hard-coded
+       "en-US".
+  +/
   string _meta_xml = format(q"┃<?xml version="1.0" encoding="UTF-8"?>
 #+END_SRC
 
@@ -2555,7 +2561,7 @@ string meta_xml(M)(
     <meta:generator>%s</meta:generator>
     <meta:creation-date>%s</meta:creation-date>
     <dc:date>%s</dc:date>
-    <dc:language>en-US</dc:language>
+    <dc:language>%s</dc:language>
   </office:meta>
 </office:document-meta>
 #+END_SRC
@@ -2564,8 +2570,9 @@ string meta_xml(M)(
 #+BEGIN_SRC d
 ┃",
 doc_matters.generator_program.name_and_version,
-doc_matters.generated_time,
-doc_matters.generated_time,
+doc_matters.modified_utc,
+doc_matters.modified_utc,
+doc_matters.src.language_tag,
 );
   return _meta_xml;
 }
diff --git a/org/out_xmls.org b/org/out_xmls.org
index db031e2..37daadf 100644
--- a/org/out_xmls.org
+++ b/org/out_xmls.org
@@ -2851,6 +2851,17 @@ string _epub_publication_uuid(string _doc_uid) {
 
 #+NAME: output_epub3_last_modified_time
 #+BEGIN_SRC d
+/+ ↓ the last modified time EPUB3 requires, exactly one dcterms:modified on
+     the package in UTC to the second, comes from doc_matters.modified_utc.
+     Set in spine rather than here because an odt's office:meta needs the
+     same instant, and a single definition cannot drift apart.
++/
+#+END_SRC
+
+**** time unused
+
+#+NAME: output_epub3_last_modified_time_UNUSED
+#+BEGIN_SRC d
 /+ ↓ the last modified time, as EPUB3 requires it: exactly one dcterms:modified
      on the package, in UTC to the second, CCYY-MM-DDThh:mm:ssZ
      .
@@ -3013,7 +3024,7 @@ string epub3_oebps_content(D,P)(D doc, P parts) {
       ? "" : xhtml_format.special_characters_plain_date(doc.matters.conf_make_meta.meta.date_published),
     (doc.matters.conf_make_meta.meta.rights_copyright.empty)
       ? "" : xhtml_format.special_characters_plain(doc.matters.conf_make_meta.meta.rights_copyright),
-    _epub_modified_utc(doc.matters.conf_make_meta.meta),
+    doc.matters.modified_utc,
     _epub_a11y_metadata(doc),
     (pth_epub3.fn_oebps_css).chompPrefix("OEBPS/"),
   );
diff --git a/org/spine.org b/org/spine.org
index c93e334..aa8b6df 100644
--- a/org/spine.org
+++ b/org/spine.org
@@ -2056,6 +2056,78 @@ template spineDocMatters() {
         }
         return Prog_();
       }
+      /+ ↓ when the document is from, as a UTC instant, for the output formats
+           that must carry a timestamp: epub3 requires exactly one
+           dcterms:modified on the package, and an odt carries
+           meta:creation-date and dc:date.
+           .
+           Taken from the document rather than from the clock, so that those
+           artefacts are a function of their source. Stamped from the clock,
+           two runs over unchanged markup produced different bytes, and no
+           comparison of that output could tell a real change from the
+           seconds between two builds.
+           .
+           Read in order: SOURCE_DATE_EPOCH where the environment sets it,
+           which is the reproducible-builds convention and lets a build pin
+           every artefact of one run; then the document's own date.modified;
+           then date.published. The clock is the last resort, for a document
+           that says nothing about when it is from.
+           .
+           A document date is loose markup: "1991", "2006-03" and
+           "2021-12-00" all occur in the sample set, and a month or day of 00
+           is not a date. A missing or zero part becomes 01, and a day out of
+           range for its month falls back to the first, so what is written is
+           always a valid CCYY-MM-DDThh:mm:ssZ.
+      +/
+      string modified_utc() {
+        import std.datetime : Date, SysTime, unixTimeToStdTime;
+        import std.format   : format;
+        import std.process  : environment;
+        import std.regex    : ctRegex, matchFirst;
+        /+ ↓ a build may pin every artefact of the run +/
+        string _sde = environment.get("SOURCE_DATE_EPOCH", "");
+        if (_sde.length > 0) {
+          try {
+            auto _t = SysTime(unixTimeToStdTime(_sde.to!long), UTC());
+            return format("%04d-%02d-%02dT%02d:%02d:%02dZ",
+              _t.year, _t.month.to!int, _t.day, _t.hour, _t.minute, _t.second);
+          } catch (Exception) {
+          }
+        }
+        /+ ↓ what the document itself says +/
+        auto _rgx_date
+          = ctRegex!(`^\s*(?P<y>\d{4})(?:[-/](?P<m>\d{1,2}))?(?:[-/](?P<d>\d{1,2}))?`);
+        foreach (_said; [
+          conf_make_meta.meta.date_modified,
+          conf_make_meta.meta.date_published,
+        ]) {
+          if (_said.length == 0) {
+            continue;
+          }
+          if (auto _m = _said.matchFirst(_rgx_date)) {
+            int _y  = _m["y"].to!int;
+            int _mo = (_m["m"].length > 0) ? _m["m"].to!int : 1;
+            int _d  = (_m["d"].length > 0) ? _m["d"].to!int : 1;
+            if (_mo < 1 || _mo > 12) {
+              _mo = 1;
+            }
+            if (_d < 1) {
+              _d = 1;
+            }
+            try {
+              cast(void) Date(_y, _mo, _d);
+            } catch (Exception) {
+              _d = 1;
+            }
+            return format("%04d-%02d-%02dT00:00:00Z", _y, _mo, _d);
+          }
+        }
+        /+ ↓ the document says nothing: the run's own time +/
+        auto _now = Clock.currTime(UTC());
+        return format("%04d-%02d-%02dT%02d:%02d:%02dZ",
+          _now.year, _now.month.to!int, _now.day,
+          _now.hour, _now.minute, _now.second);
+      }
       auto generated_time() {
         auto _st = Clock.currTime(UTC());
         auto _time = _st.year.to!string