So from my understanding, the Lockbox itself has an algorithm that changes the code automatically at 4 am, 8 am, 12 pm, 4 pm, 8 pm & 12 am. Once you have set up your account you can "generate" the code from your app remotely. The annoying part is if you have a guest whose ETA is between 3-5, that means you would have to give them 2 codes because there will be a code that works 12-4 then it will change to a different code for 4-8.
To oversimplify, it has a list of codes built-in that it changes every 4 hours, but the app can look up and tell you what code will work at that time, but you can't set or change what the code is or what time it will be active for.