Problems with dates (bug?)

I seem to have run into something that I can't explain away logically... so obviously it is a bug, right! :) What I have is a date stamp in the form YYYYMMDD. I am breaking that up and converting it to a timestamp via mktime(), adding 86400 seconds to it (1 day) and then converting it back to YYYYMMDD. This seemed easier then adding/subtracting the days, checking to see if I should wrap to 01 and advance the month, then check to see if I need to wrap the month to 01 and advance the year. But maybe not... The problem I am having is that 19991031 doesn't advance! Convert it to a timestamp, add 86400, then convert it back to a YYYYMMDD time stamp. It is still 19991031 according to 2 servers I have run this on! Adding 86400 to any other day seems to work, except this one. I found that if I crank the value up to 90000 it will work, but that starts skipping days when added to other timestamps. Please check out this example program below... call it up in a browser and see what it does (I would actually be interested to find out that it does something different on your machine). <?php $firstDate = "19991031"; echo "<B>First Date:</B> " . $firstDate . "<BR>\n";; $year = substr($firstDate, 0, 4); $month = substr($firstDate, 4, 2); $day = substr($firstDate, -2); $firstTime = mktime(0, 0, 0, $month, $day, $year); echo "<B>First Time:</B> " . $firstTime . "<BR>\n"; echo "adding 86400 seconds...<BR>\n"; $secondTime = $firstTime + 86400; echo "<B>Second Time:</B> " . $secondTime . "<BR>\n"; echo "<B>Second Date:</B> " . date("Ymd", $secondTime); ?> I've seen no other option at this point then to double check the value I get, making sure it is not equal to the last date stamp, and make changes manually. Am I missing something so obvious that I haven't looked for it, or have I really hit some mysterious wall in PHP? Many thanks for any help! Nick