From Olivier de Jonge:
Google Calendar Invoice Creator is an Adobe AIR application that reads an RSS feed from your Google Calendar in which every created appointment the number of hours worked for a project represents. Based on the title it filters the appointments, calculates the total of hours, and creates your invoice in html or text version. You need a Google account to use this application.
Version 1.8beta includes no month pattern needed anymore(uses Google data full format) and update service implented.