I am not sure this is possible today. At least, it is not easy. You can not detect a state change easily with Java Script. We did a workshop around JavaScript: https://jazz.net/library/article/1093 Lab 5.

getWorkFlowAction() also does not tell you that the state is changing. The problem is not detecting the state, it is detecting the change. I think you would have to use an attribute to calculate the current state and store it there. And then use another calculation to detect the state has changes and store the time in another attribute.

The method below won't work as it only works in conditions and it does only indicate that the state might change, but not that it actually changed.

Since 4.0:
since 4.0 M1 getWorkflowAction() returns a string that is the id of a workflow action currently selected by the user. This action will be executed when the item is saved. If the user has not selected any action this method will return null . Note: This method works only when used inside a Condition script, the return value will always be null when this is used in other customization script types.

You have way better handling on this in a participant. E.g. see http://rsjazz.wordpress.com/2012/11/27/resolve-parent-if-all-children-are-resolved-participant/

If you want to explore that route, start here: http://rsjazz.wordpress.com/2013/02/28/setting-up-rational-team-concert-for-api-development/

You also would likely need an attribute for every state you want to track.

If others solved this problem, I would be happy to hear about it.