Gleitzeit, flexitime, is the most widespread form of flexible working hours in Germany. Employees must be present only during the Kernarbeitszeit, the core time. Before and after it, they choose, within certain limits, when to start and when to stop. Who invented it is disputed. English sources credit the management consultant Christel Kammerer. In Germany, the entrepreneur Wilhelm Haller, founder of the time-recording company Interflex, is called the Vater der Gleitzeit, the father of flexitime. He took up flexible working hours at the end of the 1960s and promoted the idea with the half-Swabian, half-English slogan Ai laik Gleitzeit.