MyFile = f;
SaveFolder = Server.MapPath("\") + "returns\";
if(!System.IO.Directory.Exists(SaveFolder) )
{
    System.IO.Directory.CreateDirectory(SaveFolder);
}
MyFile.SaveAs(SaveFolder + "2011" + "000-00-0000" + ".xlsx");

MyFile = f;
SaveFolder = Path.Combine(Server.MapPath("\"), "returns");
System.IO.Directory.CreateDirectory(SaveFolder);
MyFile.SaveAs(SaveFolder, "2011000-00-0000.xlsx");

SaveFolder = Path.Combine(Server.MapPath(@""),"returns");