This element is calculated by taking the difference between
SLD_ONLINE.RUNLOG.RUN_STOP and SLD_ONLINE.RUNLOG.RUN_START.
For a given day, it can get larger than one since the
daily summation bins runs based only on their start time.
This element is best studied by summing over many days.
The data is tagged as invalid if SLD_ONLINE.RUNLOG.RUN_STOP
is set to 01-JAN-1990;00:00:00.
(description supplied by J.Perl)
The data is read from Oracle SLD_ONLINE.RUNLOG.RUN_LENGTH
