Percentage of work days in a month
64-bit Perl, 67 68

Perl 5.10 or later, run with perl -E 'code here' or perl -M5.010 filename

map{$d++,/^S/||$w++if$_=`date -d@ARGV-$_`}1..31;say int.5+100*$w/$d


Concessions to code size:


locale-sensitive: it counts as work days the days whose date output don't start with a capital S.  Run under LC_ALL=C if in doubt.
output is pure and well-formatted, but there's "garbage" on stderr on months shorter than 31.  2&gt; /dev/null if upset.
for some reason, my version of date considers 2817-12 an invalid month.  Who knew, GNU new apocalypse is due! Requires a 64 bit build of date for dates after 2038.  (Thanks Joey)