How do I change renewal dates in membership?

I have set up Membership and added all of the members but when I add a subscription to their account it sets the renewal date as the date it is when I add the subscription. I need to set all renewal dates to a certain date like September 1 for all members. How can I do this? Also, can I set Membership up to allow members to pay membership in 2 different payments one by September 1 and the 2nd half by January 1?

Thanks!