<?php

$First_Year = 1997; // change this to the first year of YOUR COMIC

// dest must be set to null to insure that the loop will continue until an actual valid
// comic date is found.
$dest = '';

// this count is a performance precaution that will probably never come up
$count = 0;

// this loop finds a valid comic.
do
{
// the performance precaution continues
$count++;

// Randomly generate numbers for day, month, and year. Year is set to never go beyond the
// current year by virtue of 2009 tag. 
$rDay = rand(1, 31);
$rMonth = rand(1, 12);
$rYear = rand($First_Year, 2009);

// Make certain rDay is properly formatted
if (strlen($rDay) < 2)
{
$rDay = '0'.$rDay;
}

// Make certain rMonth is properly formatted
if (strlen($rMonth) < 2)
{
$rMonth = '0'.$rMonth;
}

// Combine the strings for the file_exists() function. Change .html if your archives have
// a different file extension.
$rFile = 'd/' . $rYear . $rMonth . $rDay . '.html';

// The highest count ever got in tests was 6. If count reaches a number as high as 1000,
// something simply must be wrong. to prevent whatever is wrong from resulting in an
// endless loop, this will set the destination to the newest comic.
if (count > 1000){$dest = '/d/20091113.html';}

// Check to see if the file exists and set dest to end the loop if it does. The / is added at
// this point as opposed to before because having it already on the string while checking for
// the file with file_exists() seemed to keep it from working in another file of mine.
if (file_exists($rFile)){$dest = '/'.$rFile;}

} while ($dest == '');

// This opens up the page with the comic.
header('Location: '.$dest);

?>

The code for it, sans comments, is here:

<?php

$dest = '';

$count = 0;

do
{

$count++;

$rDay = rand(1, 31);
$rMonth = rand(1, 12);
$rYear = rand(2002, 2009);

if (strlen($rDay) < 2)
{
$rDay = '0'.$rDay;
}

if (strlen($rMonth) < 2)
{
$rMonth = '0'.$rMonth;
}

$rFile = 'd/' . $rYear . $rMonth . $rDay . '.html';

if (count > 1000){$dest = '/d/20091113.html';}

if (file_exists($rFile)){$dest = '/'.$rFile;}

} while ($dest == '');

header('Location: '.$dest);

?>