### 8.71 `DTIME` — Execution time subroutine (or function)

Description:
  `DTIME(VALUES, TIME)` initially returns the number of seconds of runtime since the start of the process's execution in TIME. VALUES returns the user and system components of this time in `VALUES(1)` and `VALUES(2)` respectively. TIME is equal to `VALUES(1) + VALUES(2)`.

  Subsequent invocations of `DTIME` return values accumulated since the previous invocation.

  On some systems, the underlying timings are represented using types with sufficiently small limits that overflows (wrap around) are possible, such as 32-bit types. Therefore, the values returned by this intrinsic might be, or become, negative, or numerically less than previous values, during a single run of the compiled program.

  Please note, that this implementation is thread safe if used within OpenMP directives, i.e., its state will be consistent while called from multiple threads. However, if `DTIME` is called from multiple threads, the result is still the time since the last invocation. This may not give the intended results. If possible, use `CPU_TIME` instead.

  This intrinsic is provided in both subroutine and function forms; however, only one form can be used in any given program unit.

  VALUES and TIME are `INTENT(OUT)` and provide the following:

  | `VALUES(1)`: | User time in seconds.
  | `VALUES(2)`: | System time in seconds.
  | `TIME`: | Run time since start in seconds.

Standard:
  GNU extension
Class:
  Subroutine, function
Syntax:

  `CALL DTIME(VALUES, TIME)`.
  `TIME = DTIME(VALUES)`, (not recommended).

Arguments:

  VALUES | The type shall be `REAL(4), DIMENSION(2)`.
  TIME | The type shall be `REAL(4)`.

Return value:
  Elapsed time in seconds since the last invocation or since the start of program execution if not called before.
Example:

                program test_dtime
                    integer(8) :: i, j
                    real, dimension(2) :: tarray
                    real :: result
                    call dtime(tarray, result)
                    print *, result
                    print *, tarray(1)
                    print *, tarray(2)
                    do i=1,100000000    ! Just a delay
                        j = i * i - i
                    end do
                    call dtime(tarray, result)
                    print *, result
                    print *, tarray(1)
                    print *, tarray(2)
                end program test_dtime

See also:
  CPU_TIME