Most bizarre date problem ever

Check this out: I'm returning a list of the last 30 days, looping through i, subtracting it from $end_date where $end_date is 2004-04-10 00:00:00. I'm just trying to derive a timestamp $check_date for each iteration, like 1081321200. Here's the code within the loop: $check_date = mktime(0, 0, 0, substr($end_date, 5, 2), substr($end_date, 8, 2) - $i, substr($end_date, 0, 4), -1); Note that this works PERFECTLY for every date, and always has. Except for one particular day. When $end_date - $i is supposed to be April 4, the timestamp returned is -7262, which it thinks is 12/31/1969. Can somebody PLEASE tell me how the above code manages to produce -7262, when it's always worked properly for every other day in history? Confused, - Brian