We are planning to add support for executing tasks (such as running SQL commands, stored procedures, etc.) against additional databases in the near future.
On the back end, we don't consider adTempus to be a "database" application: data storage and manipulation are not its function, and most users will never need or want to interact with the database. Our goal is for the database component of the application to be as transparent to the user as possible. We therefore chose Microsoft SQL Server Express, which we can distribute freely with the application, and install, configure, and maintain as part of the adTempus installation without the user needing to know anything about databases.
Supporting additional databases would place additional demands on us in terms of both software development and technical support, and we believe that our resources are better focused on the core mission of the software, which is scheduling.