Try
VB
Protected Sub sostxt_TextChanged(ByVal sender As Object, ByVal e As EventArgs)
Dim dt As DateTime = Convert.ToDateTime(sostxt.Text)
tostxt.Text = dt.AddDays(1).ToShortDateString()
End Sub
C#
protected void sostxt_TextChanged(object sender, EventArgs e)
{
DateTime dt = Convert.ToDateTime(sostxt.Text);
tostxt.Text = dt.AddDays(1).ToShortDateString();
}