A problem with asp.net output caching

Question

I am new in ASP.NET, currently I am learning about output caching. So at my page's Page_Load event I have assigned current time to Label control

myLabel.Text = DateTime.Now.ToString();   

and declared page directive :

<%@ OutputCache Duration="30" VaryByParam="None" %>

But then when I refresh the page, the time displayed on the label is also refreshed. Is it a correct behavior? As I understand, the page must be cached and returned the cached copy without time changing.