Just go to create a new post, as you normally would. That is how the date normally looks, but you see that 'change' next to the time? Please hit that.
Now you have this, where you can adjust the date and time.
As soon as you put the cursor in the date box to change the day a calendar pops up to select the date (or to even change the month).
Once you've selected your date, change the time. Please remember, time must be in 24 hour increments. As in 6pm must be as 18:00. I guess that's military time?
Do your post as normal, then hit post and you get this.
If later on you want to edit your post, change something, etc. You can find your scheduled posts when you hit your 'journal' tab.