When editing a post, you’ll see a button in the lower left called Post Options. Clicking on it expands variety of options for this particular post.
The first option is Publish, and by default is set to “Immediately”. Select “Scheduled Time” if you want this post to go out in the future.
This will allow you to select the date and time you wish.
With this done, you should notice that the Publish button in the upper right of the screen now says Schedule. Click that button and your post will be immediately scheduled to go out at the date and time you selected.
If you want to edit the post (or change the schedule time) before it goes out, you'll be able to access it via the Schedule button that will now appear at the bottom of the blog editor.
Please note that while you can publish to a date in the past, time travel is still being beta tested, so please keep an eye out for temporal paradoxes.