DifferentialFormatting formatting2 = new DifferentialFormatting();

ContainsBlanksRule rule2 = new ContainsBlanksRule(formatting2);
rule2.Formatting = formatting2;

ConditionalFormatting conditionalFormatting2 = new ConditionalFormatting(rule2);
worksheet.Cells[1, 2, 12, 2].AddConditionalFormatting(conditionalFormatting2);

A possible workaround is to add formatting to the rule, which copies the default:

DifferentialFormatting formatting2 = new DifferentialFormatting();
formatting2.Fill = new PatternFill(PatternType.Solid, Colors.Transparent, Colors.Transparent);
CellBorder border = new CellBorder(CellBorderStyle.Thin, new ThemableColor(Color.FromRgb(212, 212, 212)));
formatting2.LeftBorder = border;
formatting2.TopBorder = border;
formatting2.RightBorder = border;
formatting2.BottomBorder = border;

int columnCount = worksheet.UsedCellRange.ColumnCount;

for(int i = 1; i < columnCount; i++)
{
    ColumnWidth colWidth = worksheet.Columns[i].GetWidth().Value;
    if(colWidth.Value != SpreadsheetDefaultValues.DefaultColumnWidth)
    {
        ColumnWidth previousColWidth = worksheet.Columns[i - 1].GetWidth().Value;

        if(colWidth.Equals(previousColWidth))
        {
            worksheet.Columns[i].SetWidth(new ColumnWidth(colWidth.Value + 1, colWidth.IsCustom));
        }
    }
}

Workbook workbook = new Workbook();
Worksheet ws = workbook.Worksheets.Add();
ws.Cells[0, 1].SetValue("2/1/2013");

ws.Names.Add("MyField", "=Sheet1!$B$1", new CellIndex(0, 0), "My Field");

string value = ws.Names["MyField"].Value;

CellRange range;
bool success = NameConverter.TryConvertCellRangeNameToCellRange(ws.Names["MyField"].RefersTo, out range);

CellValueFormat format = ws.Cells[range].GetFormat().Value;
string result = ws.Cells[range].GetValue().Value.GetResultValueAsString(format);