Returns a double-precision scalar containing
the number of elapsed seconds corresponding to starting
at the clock valuec1 and ending at the clock value c2.
Here a clock value is a six element row vector containing the
year, month, day, hour, minute and second in that order.
If in Mlmode
c1 = clock;
c2 = clock;
O-Matrix will print a number close to 3.
If the year or the month specified by c1
do not equal the corresponding values in c2,
an error will result and the program will halt.