The docs at http://confluence.atlassian.com/display/JIRA/Advanced+Searching?clicked=jirahelp#AdvancedSearching-Created say

Use the date format that has been set by your JIRA administrator

This isn't actually the case. JQL uses a set of hard coded formats to verify against and only those are accepted. Right now the list is:

yyyy/MM/dd HH:mm
yyyy-MM-dd HH:mm
yyyy/MM/dd
yyyy-MM-dd

We should probably have some example queries to show what we mean by these.

All of the date related fields will need to have their explanation changed.