codice:
<%@ Page Language="C#" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<script runat="server">
    protected DateTime DataInizio { get; set; }
    protected DateTime DataFine { get; set; }
    
    protected void Page_Load(object sender, EventArgs e)
    {
        string di = Request["DataInizio"];
        if ((string.IsNullOrEmpty(di)))
        {
            DataInizio = DateTime.Today;
        }
        else
        {
            DataInizio = Convert.ToDateTime(di);
        }

        string df = Request["DataFine"];
        if ((string.IsNullOrEmpty(df)))
        {
            DataFine = DataInizio.AddDays(7);
        }
        else
        {
            DataFine = Convert.ToDateTime(df);
        }

    }
    
</script>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <input type="text" name="DataInizio" value="<%=DataInizio.ToString()%>"  />
        <input type="text" name="DataFine" value="<%=DataFine.ToString()%>"  />
    
    </div>
    </form>
</body>
</html>