Place the controls in a
div
<div>
<asp:button id="button1" text="Button" runat="server" onclick="button1_Click" xmlns:asp="#unknown" />
<asp:label id="label1" text="Label" runat="server" xmlns:asp="#unknown" />
</div>
Code behind: on the click event assign the date to the label control's Text Property.
protected void button1_Click(object sender, EventArgs e)
{
label1.Text = DateTime.Now.ToString();
}