<p>I have a control that is basically functioning as a client-side timer countdown control.</p>

<p>I want to fire a server-side event when the count down has reached a certain time.</p>

<p>Does anyone have an idea how this could be done?</p>

<p>So, when timer counts down to 0, a server-side event is fired.</p>

When you render the page create a client-side button that would do the action you want on postback. Then use
ClientScriptManager.GetPostBackEventReference
passing in the control as reference and add a client-side event to it using attributes as the example at the bottom of that link shows.
You can then see the Javascript it renders and use that in your function to trigger the correct server-side event.