You don't need a full blown time tracking tool which is hard to configure and only tells half the truth?
Then try this timetracker. It simply asks you every 30min what you are doing to let you enter a short note, including the date and the time difference to the previously entered item.
Based on bootstrap and jQuery, storing all data in your browser's local storage. The 30min popup is done using html5 notifications.
Usage
Open the timetracker.html file and confirm the permissions for the Nofications in your browser. Set the desired time interval on the top right corner and wait until the next full hour or half hour is due.