As others have said, you can check in before 1 (or 3) PM, just be aware depending upon your preference, how busy the fort is, whether you are willing to "Wait" for your ideal site, etc... You may not get the desired site you want. If you don't really care that it's not in the "Loop" you wanted and you just want to get in and get setup (i.e compromise), then you might get to the site sooner. If not then you may have to wait until the normal checkin time. Just KNOW before you GO what you might be dealing with.
The one year we went and we checked in about 730 AM. Took about 30 minutes to go through the process and such. The Overflow lot was PACKED. Not an empty spot to be found. I parked blocking a few spots but someone was staying with the vehicle at all times. DW and I finished checking in, we were going back to the camper to get our breakfast (just some cereal) and I no sooner poured my bowl of cereal, and I got the text message that the site was ready. (This after a security guy on a golf cart came by to comment on our parking. I told him that we would stay with the vehicle to move it if one of the blocked vehicles needed to get out. He laughed and commented on how crazy full the lot was and then went on his way).
We made it to our site with plenty of time to get set up and have a relaxing day at DTD.
Either way, I hope you enjoy your stay.