How to change whisper's time space

Hello all,

I'm working a project about using whisper with Openstack ceilometer. I implemented a backend driver for ceilometer to write metrics that it collects to whisper. It is working fine. But i have a problem with this, i need to migrate mongodb data to whisper. there aren't any tools which do it, so i must implement it myself. My problem is, when i create a whisper file, whisper's timestamps are beginning from when i create it. But my mongodb datas' timestamps are old, i need to create a whisper file which starts mongodb metrics time. How can i configure whisper to create a whisper file from a specific date ?

Thanks for your helps.