I guess you want to show Current DateTime like a Clock in TextBox. You need not to have "for loop" for this.
Try as below code. It should fullfill your requirement. It will show current DateTime in "TextBox1" after every 1 second.
protected System.Timers.Timer _timer;
protected void Page_Init(object sender, EventArgs e)
{
_timer = new System.Timers.Timer(1000);
_timer.Elapsed += new System.Timers.ElapsedEventHandler(timer_Elapsed);
}
protected void Page_Load(object sender, EventArgs e)
{
_timer.Start();
}
private void timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
{
TextBox1.Text = DateTime.Now.ToLongTimeString();
}