You can schedule posts to go out in the future by using the Set Date and Time feature.
To schedule posts out into the future, from the post page, click the Set Date and Time link. You'll see a calendar with today outlined with a box.
Choose the date you wish your message to post, along with the time. The default time will be for the next full hour. For example, if you're writing your post at 2:40 pm, the default time will be for the post to go out at 3:00 pm. Here you can see I've selected my message to post on April 9th at 10am PST. The time zone is the same as the time zone you have set under Settings - Email Settings. You can also select to send yourself an email confirmation when the message is posted. Check the check box, and then enter your email address. Once you're happy with your scheduled post time, click Save Changes.