This Extension:

Prorates membership dues based for fixed roll over date memberships based on when the user is signing up so they pay for the precentage of the year they will be a member for or if it is after the rollover date the percentage of this year plus next year. See the test file for an example

To set a price field to be prorated:

Go to the Administration menu for CiviCRM Administer-> CiviMember->Manage Price Sets Then click "View and Edit Price Fields" for the relevant Set Title Then click "Edit Price Field" for the field you would like to be prorated and click the "Prorate this price field?" checkbox.