Coding in progress is a single page application that you can use to monitor how much of the time you are in the zone when coding.
Feel free to try the app out here: https://coding-in-progress.herokuapp.com/
Click the Start button when you start getting into some serious coding activity, and hit Stop when you get interrupted.
Once you have accumulated some stats you can start digging into the report page.
You can adjust the settings used to calculate the reports on the third tab.
Each period of activity is stored in local storage. There is no back-end to this application, all the data is stored in your browser.