CRM 2011 - Calculate time spent and returning it to a attribute in the form

I've found several ways to do stuff that’s like this, but nothing works...

Right now I've have trouble just do set/update a form attribute.

Here my code:

using System;
using Microsoft.Xrm.Sdk;
using System.ServiceModel;
using Microsoft.Xrm.Sdk.Query;
using Microsoft.Crm.Sdk.Messages;

namespace inexPlugins
{
    public class TimeSpentOnCase: IPlugin
    {
        public void Execute(IServiceProvider serviceProvider)
        {
            //Extract the tracing service for use in debugging sandboxed plug-ins.
            ITracingService tracingService = (ITracingService)serviceProvider.GetService(typeof(ITracingService));

            // Obtain the execution context from the service provider.
            IPluginExecutionContext context = (IPluginExecutionContext)serviceProvider.GetService(typeof(IPluginExecutionContext));

            if (context.InputParameters.Contains("Target") && context.InputParameters["Target"] is Entity)
            {
                Entity entity = context.InputParameters["Target"] as Entity;

                if (entity.LogicalName != "incident")
                {
                    return;
                }

                try
                {
                    // Obtain the organization service reference.
                    IOrganizationServiceFactory serviceFactory = (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
                    IOrganizationService service = serviceFactory.CreateOrganizationService(context.UserId);

                    if (entity.Attributes.Contains("new_cField"))
                    {
                        entity.Attributes["new_cField"] = 999;
                        service.Update(entity);
                    }
                }
                catch (FaultException<OrganizationServiceFault> ex)
                {
                    throw new InvalidPluginExecutionException("An error occurred in the plug-in for calculating time spent on a case.", ex);
                }

                catch (Exception ex)
                {
                    tracingService.Trace("TimeSpentOnCase: {0}", ex.ToString());
                    throw;
                }
            }
        }
    }
}

/ inexHalSoul