You'll be able to book from check-in day through checkout day. After that, you'll have a rolling 60-day window if you are staying offsite after your onsite stay.
To get specific times, first choose and confirm one of the options
MDE posts for you. Choose one that is as opposite of what times you really desire.
Then, using the app, go in to Modify > Change and select a ride that you want to change the time. The app will show you the times that are available. Choose a time and Confirm.
You'll need to do this separately for each ride you want to change.