Open Source Web App For Scheduling Tweets: Tampon
For anyone willing to share tweets when they are not online, various web services exist to accomplish this task (I had also built an open source app: Scheduled Tweets -not updated anymore-).
It is a Backbone.js application that uses MongoDB for storing the data and has a PHP-based REST API.
The application allows creating a schedule with multiple entries and auto-sends your tweets at those periods.
Tweets entered can be re-ordered with drag 'n' drops or, optionally tweeted instantly.
Considering it is open sourced, Tampon can also be a good starting point for creating a feature-rich, customized version.