Article: An important idea in the definition of a university is the notion of academic freedom. The first documentary evidence of this comes from early in the life of the first university. The University of Bologna adopted an academic charter, the Constitutio Habita, in 1158 or 1155, which guaranteed the right of a traveling scholar to unhindered passage in the interests of education. Today this is claimed as the origin of "academic freedom". This is now widely recognised internationally - on 18 September 1988, 430 university rectors signed the Magna Charta Universitatum, marking the 900th anniversary of Bologna's foundation. The number of universities signing the Magna Charta Universitatum continues to grow, drawing from all parts of the world.

Question: On what date was the Magna Carta Universitatum signed?
18 September 1988