This is something to discuss with the mentor for the project (which you did and they said that it's okay) so if selected as a GSoC contributor, you can start the project during the community bonding period. Some people do this every year as they know they will have to be out for 2-3 weeks later during the coding period and they want to get started early.
As for the evaluation dates, that's up to your mentor to decide if they want to adjust them or keep them to the standard evaluation deadlines based on a 12 week project period.