For the UK to leave the EU it had to invoke Article 50 of the Lisbon Treaty which gives the two sides two years to agree the terms of the split. Theresa May triggered this process on 29 March, 2017, meaning the UK is scheduled to leave at 11pm UK time on Friday, 29 March 2019. A European court has ruled that the UK can decide to halt the process and stay in the EU at any time up to the deadline. Alternatively the process can be extended if all 28 EU members agree. But at the moment all sides are focusing on that date as being the key one, and Theresa May has put it into British law.