Are Post 9/11 GI Bill Benefits Back-dated?

That depends on what you mean by back-dated.

If you are asking whether you can use the benefits to pay for debts incurred for classes you have already taken, then no. You can only use the Post 9/11 GI Bill to pay for courses you are currently taking.

If you are asking whether you can be eligible for the new GI Bill due to military service performed before the bill was put into effect in August of 2009, the answer is yes. Any active-duty military service that falls after September 11, 2001 counts toward eligibility for the new bill.