The return date doesnt show up on your passport. Only on your flight tickets. Your passport will show a stamp of when you got to Japan, and when you are supposed to leave. By the way, dont stay past when you are supposed to leave, I have heard that they have a Huge problem with that and dont take kindly to it. They may question you as to why you are in Japan, what you intend to do, and why you are returning. They even questioned me when I first got here, Im here because my husband is stationed here, so I just had to tell them I was visiting family. They just want to make sure you arent here to work illegally or anything like that from what I gather. If you renew your visa (by leaving and returning) to many times, then they will frown on it and require you to leave for a period of time. (also, just from what I gather). All of this is assuming that you are a US citizen, it looks like you are.
You can ask as many questions as you like! I dont mind at all. I was freaking out before I came here, I am not on my husbands orders because we just got married not to long ago, so I had to come here whats called "unnacompanied" meaning no military sponsorship. I had to figure out how to do everything just like how you are. Except I didnt have a place to stay when I started, because he lives in the barracks. I know how nervewrecking it is and if you situate things ahead of time, it makes it TONS easier when you get here. Do know that things WILL fall into place when you get here though, it makes it a lot easier actually being here. I have searched these forums and they have been so helpful! I dont know if you can find ones that I have posted on, but you might want to look them up because they will have more info from other people who helped me out.. thanks everybody, you were Great!... I searched keywords like 'renewing visa' or something like that. There is some great info on this site from people helping out. Whatever you do though, keep your chin up, because at the same time as most people are SO helpful, there are always those that will tell you you "Cant" do something for whatever reason. Just because they havent done it, doesnt mean it cant be done. "Where theres a will theres a way", right? So keep gathering your info, and get ready for a chance of a lifetime, it takes some work to get over here, but you will be so glad you did... I am!
One more thing, check with the consolates while you are there at home, they dont have the answers to everything, but they help a lot. Also, if you dont get an answer you like the first time, call again and ask someone else, sometimes they are helpful, sometimes you wont get the right info, maybe ask the question a different way.
Sorry I wrote so much, I just know what I was going through trying to soak up info before I got here, so trying to help out, hope I did.