You can ask the user to tell in which time zone he reside and save that on the server and then convert all time from local time to UTC time when storing them and convert them in the reverse direction otherwise.
I don't know for the ASP.NET calendar control. I use a jQuery calendar in my application. Also in my case, the user has to select its time zone once if he does not want to use UTC time. Once a time zone has been selected, all times are converted to that zone when I render the page and converted by to UTC when I save the page. By the way, I am using ASP.NET MVC.
Check if the ASP.NET calendar somehow allow you to specify the timezone to use. Using Google, I have found that:
http://msdn.microsoft.com/en-us/library/8k0f6h1h(v=VS.85).aspx[
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By the way, it is generally easy to find answers on the web for most problem you might encounter using Google.