Hello Don,
I just discussed this with our technical team and learned that the system gives you 90 days starting from and to exactly the time of the day when you activated it. So if you activated it at 08:25:37 in the morning, then it will expire at exactly that time also. However, the time is not actually shown to the user on the simulator screen...
And so, in discussing your question, we decided to change the system in such a way that expiration will be at the end of the day, no matter what the activation time was. In that way you don't have to worry about "squeezing" in another exam early in the morning.