Thanks guys.
i got the solution for this by creating a TextChanged event of a textbox in which i wanted to put the new changed value.
protected void TextBox1_TextChanged(object sender, EventArgs e)
{
string endDate = Request.Form[((TextBox)PlaceHolder1.FindControl("tb1")).UniqueID];
string startDate = Request.Form[((TextBox)PlaceHolder1.FindControl("tb2")).UniqueID];
TextBox txtRes = (TextBox)PlaceHolder1.FindControl("tb31");
txtRes.Text = tbc.DateDiff(Convert.ToDateTime(endDate), Convert.ToDateTime(startDate));
}