Server Side Timer in an ASP.NET

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While working on a prototype recently, I came across the need to build a timer on the server side that could track how much time a person spent between from login to logout on an ASP.NET MVC web application. Another requirement was to be able to display a timer on the client representing how long the person was logged in. Finally it was required that only one browser window be active at a time. Hence if the user logged in with same credentials on multiple browsers, the system should take inputs only from the last logged in browser. To implement these features,  I chose SignalR to establish persistent connections between the logged in user and the server and then wrote a small cache to store the session information. How much of the cache should be persisted and at what point was left open.