Implement a faster date-time serialization function.

Has the side effect of recording all date-times in XML files in UTC
instead of local time with a timezone.

libgnucash/backend/sql/gnc-sql-column-table-entry.cpp

@@ -448,8 +448,9 @@ GncSqlColumnTableEntryImpl<CT_TIME>::add_to_query(QofIdTypeConst obj_name,
     if (t64 > MINTIME && t64 < MAXTIME)
     {
         GncDateTime time(t64);
-        vec.emplace_back (std::make_pair (std::string{m_col_name},
-                                          time.format_zulu ("'%Y-%m-%d %H:%M:%S'")));
+        std::string timestr("'");
+        timestr += time.format_iso8601() + "'";
+        vec.emplace_back (std::make_pair (std::string{m_col_name}, timestr));
     }
     else
     {

libgnucash/backend/sql/gnc-transaction-sql.cpp

@@ -853,7 +853,7 @@ convert_query_term_to_sql (const GncSqlBackend* sql_be, const gchar* fieldName,
             query_date_t date_data = (query_date_t)pPredData;
 
             GncDateTime time(date_data->date);
-            sql << time.format_zulu ("%Y-%m-%d %H:%M:%S");
+            sql << time.format_iso8601();
         }
         else if (strcmp (pPredData->type_name, QOF_TYPE_INT32) == 0)
         {