On the right side of the entry is an area where it says "Publish". Your entry will auto-save as a draft after a couple of minutes so it will be hard to lose a post.
You can see on the left the options that are available. You can save as a draft or publish the post immediately by hitting the blue "Publish" button.
To schedule a post select "Edit" next to "Publish immediately". This will present a time stamp that you can adjust accordingly. It's on military time. After making a change to a future time hit "OK" and the "Publish" button will become "Schedule." Select "Schedule" and the post will be set to go live at the time you chose.
It isn't possible to turn a post live with a future time. If you try that the post will automatically become a scheduled post.