. . .
  INTEGER(kind=8) :: &
     nb_periodes_initial, & ! initial value of the counter of clock periods 
     nb_periodes_final,   & ! final value of the compter of clock periods 
     nb_periodes_max,     & ! maximum value of the clock counter
     nb_periodes_sec,     & ! number of clock periods per second
     nb_periodes            ! number of clock periods of the code
  REAL(kind=8) :: elapsed_time  ! real time in seconds

  ! Initialisations
  
  CALL SYSTEM_CLOCK(COUNT_RATE=nb_periodes_sec, COUNT_MAX=nb_periodes_max)
  . . .
  CALL SYSTEM_CLOCK(COUNT=nb_periodes_initial)
  . . .
  !      <<<<<<<<<<<<< part of the code to evaluate >>>>>>>>>>>>>
  . . .
  CALL SYSTEM_CLOCK(COUNT=nb_periodes_final)
  nb_periodes = nb_periodes_final - nb_periodes_initial
  IF (nb_periodes_final < nb_periodes_initial) &
          nb_periodes = nb_periodes + nb_periodes_max
  temps_elapsed   = REAL(nb_periodes) / nb_periodes_sec
  . . .