cool#16079: keep the date field in a placeholder

A date/time placeholder saved to PPTX held our own display string
"<date/time>" as literal text with no <a:fld> at all, so PowerPoint showed
that string where the original showed a date. DrawingML::GetFieldValue has
no case for the field kind a header/footer date carries, "DateTime", and
with no type name nothing writes the field.

Name the type that leaves the format to the reading application, datetime:
the field carries no format of its own, the page holds it. PowerPoint writes
datetimeFigureOut in such a placeholder, an alias of datetime1 - MS-OI29500
part 1, 21.1.2.2.4 (fld) - which would pin a formatless field to MM/DD/YYYY;
that alias is read on import as the datetime1 it stands for, so a slide's own
date field keeps its format across a round-trip. Every other datetime* type
was understood; the one PowerPoint itself writes was being dropped.

The page's own DateTimeFormat is out of reach where the type is chosen, so
a footer set to show a time is written as a date. PowerPoint has no
per-page format to fall back on either way, and a date beats the literal
string this replaces.

Signed-off-by: Mike Kaganski <mike.kaganski@collabora.com>
Change-Id: Icbd9ac57ed7b2311d5f6570fffad1cd78e4d6bf7
Reviewed-on: https://gerrit.collaboraoffice.com/c/online/+/9192
Tested-by: Jenkins CPCI <releng@collaboraoffice.com>
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>
(cherry picked from commit 145a37d84dd3b22023c46b6be8577008f36a48b9)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/210141
Tested-by: Jenkins
Reviewed-by: Xisco Fauli <xiscofauli@libreoffice.org>

oox/source/drawingml/textfield.cxx

@@ -215,7 +215,9 @@ SvxDateFormat TextField::getLODateFormat(std::u16string_view rDateTimeType)
     if( aDateTimeNum.empty() ) // "datetime"
         return SvxDateFormat::StdSmall;
 
-    int nDateTimeNum = o3tl::toInt32(aDateTimeNum);
+    // "datetimeFigureOut", which is what PowerPoint writes in a date placeholder, is an alias of
+    // datetime1: MS-OI29500 part 1, 21.1.2.2.4 (fld).
+    int nDateTimeNum = aDateTimeNum == u"FigureOut" ? 1 : o3tl::toInt32(aDateTimeNum);
 
     switch( nDateTimeNum )
     {

oox/source/export/drawingml.cxx

@@ -3031,6 +3031,12 @@ OUString DrawingML::GetFieldValue( const css::uno::Reference< css::text::XTextRa
                     rXPropSet->getPropertyValue(UNO_TC_PROP_NUMFORMAT) >>= nNumFmt;
                     aFieldValue = GetDatetimeTypeFromDate(static_cast<SvxDateFormat>(nNumFmt));
                 }
+                else if(aFieldKind == "DateTime")
+                {
+                    // The date/time field of a header/footer area carries no format of its own -
+                    // the page holds it - and "datetime" seems a perfect fit (see MS-OI29500).
+                    aFieldValue = u"datetime"_ustr;
+                }
                 else if(aFieldKind == "ExtTime")
                 {
                     sal_Int32 nNumFmt = -1;

sd/qa/unit/export-tests-ooxml4.cxx

@@ -2450,6 +2450,21 @@ CPPUNIT_TEST_FIXTURE(SdOOXMLExportTest4, testCool16078_placeholderKeepsItsOutlin
     assertXPath(pLayout, "//p:sp[p:nvSpPr/p:nvPr/p:ph/@type='pic']/p:spPr/a:prstGeom", 0);
 }
 
+CPPUNIT_TEST_FIXTURE(SdOOXMLExportTest4, testCool16079_dateTimeField)
+{
+    // Given a deck whose date placeholders hold an automatically updated date:
+    createSdImpressDoc("pptx/master-and-eleven-layouts.pptx");
+    save(TestFilter::PPTX);
+
+    // The placeholder keeps a field. Without the fix it held our own display string as literal
+    // text, so PowerPoint showed "<date/time>" where the original showed a date. The type is the
+    // one that leaves the format to the reader, which is what a field holding none asks for - the
+    // alias PowerPoint writes here stands for a fixed MM/DD/YYYY.
+    xmlDocUniquePtr pLayout = parseExportedLayoutNamed(u"Title Slide");
+    assertXPath(pLayout, "//p:sp[p:nvSpPr/p:nvPr/p:ph/@type='dt']/p:txBody/a:p/a:fld", "type",
+                u"datetime");
+}
+
 CPPUNIT_PLUGIN_IMPLEMENT();
 
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