How to extends specified module into the zimbra?

[lgmoim]

I've only utilized the zimbra for several days, and find that this web-based application is pretty good enough to used as a groupware in our company.
Now, here is the thing, I want to add specified functionality into zimbra, however, I wasn't able to find any single documentation for developer(Like API or something). As far as I concerned, zimbra(I mean web application part) was principally developed by J2EE, Ajax, and also related with other technologies.

The main functionality which I really want to add into the zimbra is attendance checking. The main scenario could be like this. The system can automatically make a record when each member login into the system, and by using this record manager can check the status of the member's attendance.

In order to implement this function, I deem I have to create proper table first, after that develop View layer by utilizing HTML & Ajax and use servlet to implement Controller layer. But I have no idea how to approach this application. Is there any sample source code posted in the forum? Or some methodologies can make me much more easily approach the development?