啊，求助，EXCEL导入datagridview, 日期显示的是一串数字。。。怎么回事啊？代码见内。
比如在EXCEL中是2011/11/11,导入到datagridview就是一串数字。
求如何在datagridview显示的也是日期啊。。。

导入Excel
private void button1_Click(object sender, EventArgs e)
{
OpenFileDialog ofd = new OpenFileDialog();
ofd.Filter = "Excel文件(*.xls)|*.xls";
if (ofd.ShowDialog() == DialogResult.OK)
{
string filename = ofd.FileName;
ExcelToDataGridView(filename);
}
}

private void ExcelToDataGridView(string filename)
{
Microsoft.Office.Interop.Excel.Application excel = new Microsoft.Office.Interop.Excel.Application();
Microsoft.Office.Interop.Excel.Workbook workbook;
Microsoft.Office.Interop.Excel.Worksheet worksheet;

object oMissing = System.Reflection.Missing.Value;

workbook = excel.Workbooks.Open(filename, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing);

worksheet = (Worksheet)workbook.Worksheets[1];

int rowCount = worksheet.UsedRange.Rows.Count;
int colCount = worksheet.UsedRange.Columns.Count;


Microsoft.Office.Interop.Excel.Range range1;

System.Data.DataTable dt = new System.Data.DataTable();

for (int i = 0; i <= colCount; i++)
{
try
{
range1 = worksheet.get_Range((object)worksheet.Cells[1, i + 1], (object)worksheet.Cells[1, i + 1]);
dt.Columns.Add(range1.Value2.ToString());

}

catch (Exception)
{

}
}
for (int j = 1; j < rowCount; j++)
{
DataRow dr = dt.NewRow();
for (int i = 0; i <= colCount; i++)
{
try
{
range1 = worksheet.get_Range((object)worksheet.Cells[j + 1, i + 1], (object)worksheet.Cells[j + 1, i + 1]);
dr[i] = range1.Value2.ToString();
}
catch (Exception)
{ }
}

dt.Rows.Add(dr);
}
dataGridView1.DataSource = dt;
excel.Quit();
}