Use format control STRING to format the number SECONDS.
The valid format specifiers are:
%y is the number of (365-day) years.
%d is the number of days.
%h is the number of hours.
%m is the number of minutes.
%s is the number of seconds.
%z is a non-printing control flag (see below).
%% is a literal "%".
Upper-case specifiers are followed by the unit-name (e.g. "years").
Lower-case specifiers return only the unit.
"%" may be followed by a number specifying a width, with an
optional leading "." for zero-padding. For example, "%.3Y" will
return something of the form "001 year".
The "%z" specifier does not print anything. When it is used, specifiers
must be given in order of decreasing size. To the left of "%z", nothing
is output until the first non-zero unit is encountered.
This function does not work for SECONDS greater than `most-positive-fixnum'.