OfficeOpenXml.Table.PivotTable.ExcelPivotTableField.SetDateGroup C# (CSharp) Method

        internal ExcelPivotTableFieldDateGroup SetDateGroup(eDateGroupBy GroupBy, DateTime StartDate, DateTime EndDate, int interval)
        {
            ExcelPivotTableFieldDateGroup group;
            group = new ExcelPivotTableFieldDateGroup(NameSpaceManager, _cacheFieldHelper.TopNode);
            _cacheFieldHelper.SetXmlNodeBool("d:sharedItems/@containsDate", true);
            _cacheFieldHelper.SetXmlNodeBool("d:sharedItems/@containsNonDate", false);
            _cacheFieldHelper.SetXmlNodeBool("d:sharedItems/@containsSemiMixedTypes", false);

            group.TopNode.InnerXml += string.Format("<fieldGroup base=\"{0}\"><rangePr groupBy=\"{1}\" /><groupItems /></fieldGroup>", BaseIndex, GroupBy.ToString().ToLower());

            if (StartDate.Year < 1900)
            {
                _cacheFieldHelper.SetXmlNodeString("d:fieldGroup/d:rangePr/@startDate", "1900-01-01T00:00:00");
            }
            else
            {
                _cacheFieldHelper.SetXmlNodeString("d:fieldGroup/d:rangePr/@startDate", StartDate.ToString("s", CultureInfo.InvariantCulture));
                _cacheFieldHelper.SetXmlNodeString("d:fieldGroup/d:rangePr/@autoStart", "0");
            }

            if (EndDate==DateTime.MaxValue)
            {
                _cacheFieldHelper.SetXmlNodeString("d:fieldGroup/d:rangePr/@endDate", "9999-12-31T00:00:00");
            }
            else
            {
                _cacheFieldHelper.SetXmlNodeString("d:fieldGroup/d:rangePr/@endDate", EndDate.ToString("s", CultureInfo.InvariantCulture));
                _cacheFieldHelper.SetXmlNodeString("d:fieldGroup/d:rangePr/@autoEnd", "0");
            }

            int items = AddDateGroupItems(group, GroupBy, StartDate, EndDate, interval);
            AddFieldItems(items);

            _grouping = group;
            return group;
        }

        private void AddCacheField(ExcelPivotTableField field, DateTime startDate, DateTime endDate, int interval)
        {
            //Add Cache definition field.
            var cacheTopNode   = _table.CacheDefinition.CacheDefinitionXml.SelectSingleNode("//d:cacheFields", _table.NameSpaceManager);
            var cacheFieldNode = _table.CacheDefinition.CacheDefinitionXml.CreateElement("cacheField", ExcelPackage.schemaMain);

            cacheFieldNode.SetAttribute("name", field.DateGrouping.ToString());
            cacheFieldNode.SetAttribute("databaseField", "0");
            cacheTopNode.AppendChild(cacheFieldNode);
            field.SetCacheFieldNode(cacheFieldNode);

            field.SetDateGroup(field.DateGrouping, startDate, endDate, interval);
        }