Twitter is now allowing advertisers to schedule tweets in advance:

https://blog.twitter.com/2013/now-available-scheduled-tweets

It is also possible to schedule tweets in advance using outside products. This has been possible for some time, preceding Twitter's recent announcement. This article will let you know about some of the additonal products that you can use to schedule Tweets:

http://www.makeuseof.com/tag/6-free-ways-to-schedule-tweets/