'Get wall clock time between two successive counters in msecs.'

def _get_total_time(self, counters_t1, counters_t2, counter_name):

    timestamp_t1 = float(self._find_performance_counter_timestamp(counters_t1, counter_name))
    timestamp_t2 = float(self._find_performance_counter_timestamp(counters_t2, counter_name))
    return ((timestamp_t2 - timestamp_t1) * 1000.0)
