The spell clock is a new element in the NWN2 user interface. It's like a digital alarm clock intended to count spell duration (but it could also count any other time period). It could be very useful when casting long term buffing spells and want to know how much time is left before they expire.
The way the spell clock is implemented, it will work with all official or community modules / campaigns without causing, in any of them, any kind of problems or side effects or affecting anything (or be affected by anything).
The spell clock user interface is located to the right of the action queue.
The pdf file describes in detail how to use the clock. Some special issues are also answered there.
Note: If you are a module builder, you might want to try a somewhat similar mod by The Fred. You can find it here.
10 - A Masterpiece, Genuinely Groundbreaking 9 - Outstanding, a Must Have 8 - Excellent, Recommended to Anyone 7 - Very Good, Deserves a Look 6 - Good, Qualified Recommendation 5 - Fair, Solid yet Unremarkable 4 - Some Merit, Requires Improvements 3 - Poor Execution, Potential Unrealized 2 - Very Little Appeal 1 - Not Recommended to Anyone