Every time you start the image, Virtualbox is "telling" the guest OS what the current time is, so that it can sync the clock.
So even if the guest OS in your backup image file is freshly installed, whenever you start it it will expire immediately if it's gone more than the trial period. In order to avoid that, you have to stop time syncing between the host and the guest OS before starting up the image.
Check this post on SU regarding how to achieve this in Virtualbox on Ubuntu.