I have had this problem with both the 2000 and 2007 versions of excel: they won't sort dates correctly. Some of the dates will be in the correct order, then others will be randomly incorrect or all found at the end. All the dates are formatted as dates, were typed in, and are formatted the same. Here is an example. Where it is in red it suddenly changes. There are many dates before the first one of 2-Jul, but I didn't include them here, as they are properly sorted. I can't find an answer to this anywhere.
2-Jul5-Jul6-Jul10-Jul13-Jul19-Aug31-Aug9-Sep21-Sep28-Sep3-Nov5-Jan27-May29-May16-Jun7-Jul13-Jul14-Jul6-Aug6-Aug16-Oct5-Nov12-Nov8-Dec
2-Jul5-Jul6-Jul10-Jul13-Jul19-Aug31-Aug9-Sep21-Sep28-Sep3-Nov5-Jan27-May29-May16-Jun7-Jul13-Jul14-Jul6-Aug6-Aug16-Oct5-Nov12-Nov8-Dec