The university traces its roots back to at least the end of the 11th century, although the exact date of foundation remains unclear. This dating would make its duration now equal to Plato's Academy (400s BC to AD 500s). According to legend, after riots between students and townsfolk broke out in 1209, some of the academics at Oxford fled north-east to the town of Cambridge, where the University of Cambridge was founded. The two universities have since had a long history of competition with each other, and are the most selective universities in the UK. (see Oxbridge rivalry).