Is there a way to change creation and closing date for a ticket ?
Every week I export a report of all created/closed tickets and I need to change date for some of them.
Creation and closing time of a ticket should not be changed. This is the start and the end of a workflow of a ticket.
If you need to get more information in your reports, rethink the ticket's workflow. Trac provides you a workflow,
http://trac.edgewall.org/wiki/TracWorkflow
but you can customise it easily according to your needs through trac.ini.
In the case however you want to hack it, everything is in the trac database and you can change anything by hand, but don't go there.