Percentage of work days in a month Given a year and a month, find out the percentage of work days in said 
month. Work days are Monday through Friday with no regard to holidays or 
other special things. The Gregorian calendar is used.

Input

A year and month in ISO 8601 format (YYYY-MM). The year always has four 
digits, the month always has two digits. The given year will not be before 
1582.

Output

Output is the percentage of work days (according to above definition) in 
the given month, rounded to a whole number. No percent sign or fractional 
digits follow.

Sample 1

Input                Output

2010-05              68


Sample 2

Input                Output

2010-06              73


Sample 3

Input                Output

1920-10              68


Sample 4

Input                Output

2817-12              68


A week has passed, an answer has been accepted. For the curious, the sizes of the submissions we got in our contest:


  129 – Z shell
  174 – VB.NET
  222 – C
  233 – C
  300 – C


As well as our own (unranked) solutions:


    75 – PowerShell
    93 – Ruby
  112 – Bourne shell
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)