this is my solution:
-JavaScript Code
<script type="text/javascript">
var localTime = new Date();
var h = localTime.getHours();
document.getElementById("<%=Hidden1.ClientID%>").value = h;
</script>
-HTML
<asp:hiddenfield id="Hidden1" value="0" runat="server" xmlns:asp="#unknown" />
-Behind Code for C#
string localHour = Hidden1.Value.ToString();
string UTChour = DateTime.UtcNow.ToString("HH");
int diffTime = Convert.ToInt32(localHour) - Convert.ToInt32(UTChour);
Hope it will help you.,
God Bless.