I had the same question as you and sent an SM this morning. I was told they go by transaction date and not post date

This is the response I got:



I am writing in response to your inquiry about the 5% cash
back promotion.

I would like to confirm that, the 5% cash back bonus
points are based on the transaction date and not the post
date. Therefore, if a purchase is made on the last day of
the month, it will still be included for the additional
bonus points even if they dont post for a couple of days.

Further, I see that, your account is enrolled in the third
quarter promotional offer of 5% rewards for purchases
classified as Gas stations and Restaurants from July 1st
to September 30th, 2012. Rewards will post within six to
eight weeks from the purchase date. There is a maximum of
$60.00 or 6,000 reward points on the $1,500.00 spend.

If you have any further questions, please reply using the
Secure Message Center.

Thank you,

Neelu Sachdeva
E-mail Customer Service Advisor

1-800-436-7927

Account is owned by Chase Bank USA, N.A. and may be
serviced by its affiliates.