When a user exports to excel by clicking on 'All Fields' the date formats in Excel differ between issues (rows).
Some result in the general form (05/Oct/06 10:56 AM) while others use the 24hour format like so (22/09/2006 14:11:00). Note the dates are captured in American date format (MM/DD/YYYY hh:mm) but in a 'custom' cell format when exported to Excel.
This issue was originally captured in https://jira.atlassian.com/browse/JRA-11360 but is still an issue.
For more information see JSP-7554.