DateTime start = DateTime.Now;
DoSomeCall();
double secondsToLog = (DateTime.Now - start).TotalSeconds;

This is not at all necessary: using the StopWatch class. The StopWatch class is more efficient than any other solution provided or created in .NET since it uses low-level API calls and supports a high-resolution performance counter (when there is hardware and software support).

Stopwatch watch = new Stopwatch();
watch.Start();
DoSomeCall();
watch.Stop();
double secondsToLog = watch.Elapsed.TotalSeconds;