Please do not modify the non-internal part of the interface as that will result with an error when you submit your homework. (Just as the comment there says "Interfaces of the chess clock Do not modify!".
If you need any additional variables, you can create as many as you want in the internal part. Why would it be necessary to add your variable to the interface part?
The default value is -1, but you can change the value using actions or when entering/exiting states etc. With other words, create a model that changes the value according to your specification while the chess clock is on and working. But do not change the default -1 value.