Normally when you clicking the publish/update button, your post will be published immediately, but if you want to publish your post at a certain time in future, you can schedule this post’s publishing time.  In the Publish meta-box, click Edit link,

Set the date & time when you want to publish this post. Note that to mention the time in PM format you have to specify it in the 24-hour clock time format. E.g. 2 PM will be written as 14:00. Click on OK and then on Schedule button.