Two functions ETIME and DTIME. let FORTRAN programs measure time. This is useful for perfomance tuning. They are used as follows:
REAL TIMEARRAY (2)
ELAPSE = ETIME(TIMEARRAY)
ETIME returns a real number which is the total CPU time used for this process it began executing. DTIME returns a real number which is the running time for this process since the last call to DTIME. Both procedures use TIMEARRAY to return the user time and system time separately; TIMEARRAY(1) reports the user time, and TIMEARRAY(2) supports the system time. For example, the statements
REAL DELAPSE TIMEARRAY(2), X
DO 10 I=1, 100000
set DELAPSE to the time required to compute the loop DO 10 1=1,100000.
 Mike Loukides, UNIX for FORTRAN Programmers, O’ Reilly and Associates, Inc, 103 Morris Street, Sebastopol, CA 95472, (1990, 1991) see page 89