Your arrival date is known by your reservation number, which should be on the confirmation you received from your TA.
However, anyone can make ADR at 180 days. You don't need the reservation number. They will ask which resort you are staying at and whether you are on a dining plan.
You do need the number to do the 180 + 10 where you can book 180 days from your departure date (the date you are leaving WDW)...so in effect you get the extra number to equal the number of days you are staying, but no more than 10.
Was that clear as mud? If you were staying 14 days, you could still book ADRs only 190 days ahead (180 + 10). If you were staying 7 days, you could book 187 ahead (180 + 7). For this, you need the reservation number.
You can call Disney Dining and book the ADRs yourself or your TA can do it. If you are 180 days out, there should be no problems except maybe with CRT. We're going in a month, and I got everything I wanted last week. The advantage to booking yourself is that IF you can't get something you want, you know what your alternatives are--do you want to try for it on a different day? If so, what else are you willing to move around? A TA may not know all of your flexibility options.
Before I call, I make my "cheat sheet" where I list the days/dates, which parks are having extra magic hours, and which parks we plan to visit that day. Then I list my preferred ADR, and in another color, an acceptable alternative ADR. With that info in front of me, I can make any changes to my first choice plans very quickly. For instance, if I wanted CR on Tuesday, I might have listed it as an alternate for Friday. I can quickly see that those are both Epcot days on my schedule, so could flip days with any other Epcot restaurant if I needed to.