Question: The Volkswagen Concept D was envisioned by the chairman of the supervisory board until what date?

Context paragraphs:
Codetermination in Germany: Codetermination in Germany is a concept that involves the right of workers to participate in management of the companies they work for.  Known as "Mitbestimmung", the modern law on codetermination is found principally in the "Mitbestimmungsgesetz" of 1976.  The law allows workers to elect representatives (usually trade union representatives) for almost half of the supervisory board of directors.  The legislation is separate from the main German company law Act for public companies, the "Aktiengesetz".  It applies to public and private companies, so long as there are over 2000 employees.  For companies with 500-2000 employees, one third of the supervisory board must be elected.

Rijnlands Lyceum Foundation: The Rijnlands Lyceum Foundation is an educational organisation in the Netherlands encompassing seven different schools offering both primary and secondary education.  The foundation is based in Wassenaar.  The executive director of the organisation is Dr. Maarten Knoester (since 2008), the chairman of the supervisory board is Frans Weisglas (since 2010), former chairman of the Lower House of the Dutch Parliament.  As chairman of the supervisory board Mr Weisglas succeeded Hans Dijkstal, former vice-prime minister of the Netherlands.

Volkswagen Concept BlueSport: The Volkswagen Concept BlueSport is a mid-engined roadster concept car produced by Volkswagen.  It was introduced at the 2009 North American International Auto Show in Detroit.  The Concept BlueSport follows on from a previous roadster concept car, the Volkswagen Concept R, shown at the 2003 Frankfurt Motor Show.

Julie Dickson: Julie Dickson, {'1': ", '2': ", '3': ", '4': "} was the Superintendent of the Office of the Superintendent of Financial Institutions of Canada, from July 4, 2007 to June, 2014.  She later became a European Central Bank representative on the Supervisory Board of the Single Supervisory Mechanism, concerned with uniform regulatory oversight of European Banks.  The supervisory group, or "Single Supervisory Mechanism", is composed of the European Central Bank and the national supervisory authorities of participating countries.

Ferdinand Piëch: Ferdinand Karl Piëch (born 17 April 1937) is an Austrian business magnate, engineer and executive who was the chairman of the supervisory board ("Aufsichtsratsvorsitzender") of Volkswagen Group until 25 April 2015.

Shariah Board: A Sharia Board (also Shariah Supervisory Board, Advisory Board or Religious Board) certifies Islamic financial products as being Sharia-compliant (i.e. in accordance with Islamic law).  Because compliance with shariah law is the underlying reason for the existence of Islamic finance, Islamic banks (and conventional banking institutions that offer Islamic banking products and services) should establish a Shariah Supervisory Board (SSB) to advise them on whether their products comply, and to ensure that their operations and activities comply with Shariah principles.  There are also national sharia boards in many Muslim majority countries that regulate Islamic financial institutions nationwide.

Martin Winterkorn: Martin Winterkorn (born 24 May 1947) is the former chairman of the board of directors (CEO, "Vorstandsvorsitzender" in German) of Volkswagen AG, the parent company of the Volkswagen Group, and former chairman of the supervisory board of Audi and Porsche Automobil Holding SE.

Volkswagen Concept D: The Volkswagen Concept D is a concept luxury liftback envisioned by Ferdinand Piëch and unveiled by Volkswagen at the 1999 International Motor Show Germany.  It featured V10 TDI diesel engine (230 kW/313 bhp at 4000 rpm., 750 Nm torque), 6-speed Tiptronic, 4Motion all-wheel drive, air suspension with adaptive damping, and Bi-Xenon headlamps.

Christoph Kohl (chemist): Christoph Kohl (born 11 January 1900 in Frankfurt) was a German chemist and industrial leader.  He was chairman of the board of directors and CEO of the chemical and pharmaceutical company Cassella from 1964, chairman of its supervisory board and chairman of the supervisory board of Riedel-de Haën.

Rein Willems: Rein Willems (born 26 August 1945, Geleen) was President of Shell Nederland from 2003-2007.  Since 1969, he has held a number of positions at Shell, with postings in Singapore, Australia, the UK, the Philippines and Brazil.  In addition, he was a member of the Executive Committee and the Environmental Committee of the Employers Association VNO-NCW and the Innovation Platform.  He is Chairman of the Supervisory Board of Koninklijke Joh.  Enschedé, member of the Supervisory Board of the Vrije Universiteit Amsterdam, the Centraal Orgaan Opvang Asielzoekers (COA) and the Van Leeuwen Pipe and Tube Group B.V.  He was Chairman of the national Task Force Energy Transition, which was established to make the Dutch economy more sustainable.  He is a member of the supervisory board of FMO, the Dutch development bank.

Supporting paragraphs: Volkswagen Concept D Ferdinand Piëch
Answer: 25 April 2015