Here's a tweet from a serious Harold Camping website:
One of the brethren with great understanding of the Biblical calendars created a spreadsheet showing every day from May 21, 1988 to October 21, 2011 using both inclusive and non-inclusive methods of counting. This shows that the 153rd day ends on sunset October 22rd (Tishri 23) Jerusalem time.
Here is Chris McCann's explanation from 10/21/11 (mp3). McCann is currently a featured speaker on Family Radio.
"Our error was thinking May 21 to October 21 in terms of our 24 hour days starting in the morning rather than in terms of evening-mornings which would mean May 21/22 (evening/morning) to October 21/22 (evening/morning)."