From the website:
Book in advance online at www.recreation.gov to decide what time your tour begins! With a limited number of tickets available online each day, reservations fill quickly. Reservations may be made up to three months in advance. Reserve your tickets early. Reserved tickets must be picked up one hour prior to your USS Arizona Memorial tour. Unclaimed tickets will be reissued to the general public.
Hummm.

Maybe ours was a different opening period for reservations because we included the USS Missouri?! All I can say to that is that I have my confirmation printed out and lying right here in front of me right now with a reservation # and barcode dated 02/10/2014 for an 8:45 AM "Passport to Pearl Harbor" on 08/10/2014. It is for 3 confirmed adults @ $55.00 pp + the reservation fees for a total billed to my MC for $169.50. That is definitely 6 months in advance that I reserved it. I am copying it below (xx'ing out my personal info).
Reservation 4-2088324
Date: 02/10/2014
LEEANN
XXXXXXXXXX
Access your reservations online at Recreation.gov !
SIGN-IN:
___________________@hotmail.com
CUSTOMER INFO
PHONE: XXX-XXX-XXXX
NAME: LEEANN XXXXXXXXXX
ADDRESS: xxxxxxxxxxxxxxx
XXXXXXXXX, MD 20XXX
TICKET INFO
PEARL HARBOR HISTORIC SITES (USS Arizona)
Ticket Category: Individual
Tour DATE Tour NAME DELIVERY METHOD STATUS TYPE QTY RATE FEES
08/10/2014 Passport to Pearl Harbor
8:45 AM HST - USS Arizona Memorial
8:00 AM - 5:30 PM HST - Passport
3 Adult Confirmed Adult 3 $55.00 $165.00
Reservation Fee $4.50
Sub Total: $169.50
Total: $169.50
PAYMENT INFORMATION
*IMPORTANT BILLING INFORMATION: Recreation.gov - 888-448-1474 NY
PAST PAID: $ 0.00
PAYMENT TENDERED: $ 169.50
NEW AMOUNT OWING: $ 0.00
PAYMENT:
02/10/2014 MasterCard ************xxxx $ 169.50
DIRECTIONS
For driving directions to the Pearl Harbor Visitor Center, please visit http://
www.nps.gov/valr/planyourvisit.
TOUR NOTES
USS Arizona Memorial:
Tickets must be picked up one (1) hour prior to your tour. Unclaimed tickets will be reissued to walk-in traffic. Parking can be difficult so plan for traffic and allow time to find a parking spot at the Visitor Center.
Bags are prohibited at the Pearl Harbor Visitor Center due to strict security measures so please pack lightly. You may store your bags in the storage facility near the Visitor Center entrance for a $3 fee per bag., Attention all teachers and school group coordinators: Please click here and follow the link for answers to FAQs, including what is expected of our student visitors and their chaperones.
Passport to Pearl Harbor:
One-Day Passport Option: A one-day Passport to Pearl Harbor allows you to visit all four Pearl Harbor Historic Sites and includes admission to: the USS Arizona Memorial with Audio Tour , to the USS Bowfin Submarine Museum & Park, the Battleship Missouri Memorial, and the Pacific Aviation Museum. Reserved Passports to Pearl Harbor can be picked up at the Pearl Harbor Visitor Center's NPS Tickets and Information Desk.
Two-Day Upgrade Option: If you wish to extend your Passport to Pearl Harbor for a second day, you may purchase a $10 upgrade at the Visitor Center's Pearl Harbor Historic Sites' Tickets and Information Desk. The second day is valid for an additional seven days.
Please visit each site's own website for more details on planning your day(s) at Pearl Harbor.