Use a bit more standard null date

The two places in cellvalueconversion.cxx using the null date were
introduced in commit a7e949673c
(gridfixes: #i117265# use a NumberFormatter for converting cell
values to text., 2012-09-14); they are used together, so that the
code in lcl_convertDateToDays uses the same epoch as defined in
the respective number formatter, and the obtained number value is
only ever used inside convertToString in a call to the formatter's
convertNumberToString. The old value was arbitrary; and the new
value has some optimizations in the code (see Date::DateToDays and
comphelper::date::convertDateToDaysNormalizing called from it).

Change-Id: I9c050d26ace1cf3fccde579d0d91c00f26bb3fd8
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/190788
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

toolkit/source/controls/table/cellvalueconversion.cxx

@@ -60,7 +60,7 @@ namespace
 double lcl_convertDateToDays(sal_uInt16 const i_day, sal_uInt16 const i_month,
                              sal_Int16 const i_year)
 {
-    tools::Long const nNullDateDays = ::Date::DateToDays(1, 1, 1900);
+    tools::Long const nNullDateDays = ::Date::DateToDays(30, 12, 1899);
     tools::Long const nValueDateDays = ::Date::DateToDays(i_day, i_month, i_year);
 
     return nValueDateDays - nNullDateDays;
@@ -280,7 +280,7 @@ bool CellValueConversion::ensureNumberFormatter()
             = NumberFormatsSupplier::createWithLocale(xContext, aLocale);
 
         // ensure a NullDate we will assume later on
-        css::util::Date const aNullDate(1, 1, 1900);
+        css::util::Date const aNullDate(30, 12, 1899);
         Reference<XPropertySet> const xFormatSettings(xSupplier->getNumberFormatSettings(),
                                                       UNO_SET_THROW);
         xFormatSettings->setPropertyValue(u"NullDate"_ustr, Any(aNullDate));