Evvai!! ho risolto infatti! grazie!!!

incollo il codice per i posteri

<script runat="server">

protected void Trova_Data(object sender, EventArgs e)
{
DateTime DataDelFile;
DataDelFile= System.IO.File.GetLastWriteTime(Server.MapPath(Hyp erLink.NavigateUrl));
Label1.Text = FileDate.ToString("f");
}

</script>

<asp:HyperLink ID="HyperLink" runat="server" NavigateUrl="~/Default.aspx"> Default.aspx </asp:HyperLink>
<asp:Label ID="Label1" runat="server" Text="Label" OnLoad="Trova_Data" ></asp:Label>