Change from string to double and complete the subtraction

<script type="text/javascript">
        var localTime = new Date();
        var h = localTime.getHours();
        document.getElementById("<%=Hidden1.ClientID%>").value = h;
</script>

<asp:hiddenfield id="Hidden1" value="0" runat="server" xmlns:asp="#unknown" />

string localHour = Hidden1.Value.ToString();
string UTChour = DateTime.UtcNow.ToString("HH");
int diffTime = Convert.ToInt32(localHour) - Convert.ToInt32(UTChour);

using System;

public class Program
{
    public static void Main()
    {
        int UTChour = DateTime.UtcNow.Hour;
        int localhour = DateTime.Now.Hour;
        int hourDiff = localhour - UTChour;
        Console.WriteLine(hourDiff);
    }
}