It is also 82 days after Theresa May sent a letter formally notifying the European Council of Britain's intention to withdraw.
What is the timeline from now on?
- June 19 - Brexit Secretary David Davis and European Commission chief negotiator Michel Barnier open negotiations in Brussels.
Issues on the table will include the status of EU citizens in the UK and of British people living in continental Europe, as well as the Northern Irish border and the size of any "divorce bill" - estimated at anything up to 100 billion euro (£88 billion).
The EU insists that phase one of talks will focus only on the terms of withdrawal, and not on a future trade relationship.
- June 21 - The Queen's Speech sets out details of extensive UK Government legislation required to put Brexit into effect, expected to include a Great Repeal Bill bringing EU laws and regulations onto the British statute book, as well as bills on issues including immigration and customs.
- June 22 - Mr Barnier reports back to the leaders of the EU27, who are expected to endorse a process for relocation of EU agencies currently based in the UK.
The meeting comes ahead of a two-day summit of the European Council on June 22 and 23, attended by all 28 leaders including Theresa May, which is due to focus on migration and security.
- Summer - Negotiations on the Brexit deal expected to continue in Brussels, with the Davis and Barnier teams meeting for one week in each month, before returning to base to report back and develop their positions.
- September 24 - German federal elections could see Angela Merkel replaced as Chancellor by former European Parliament president and staunch federalist Martin Schulz, who once called for the creation of a ''genuine European government''.
- October 19 - Mr Barnier is set to report back to the EU27 on whether sufficient progress has been made to move on to phase two of the talks, covering the UK's future trade relationship with the remaining EU. The Prime Minister will attend a two-day European Council summit in Brussels.
- Winter/spring 2017/18 - If talks are proceeding to plan, negotiators can be expected to be meeting regularly to iron out remaining issues and identify the points of difference to be settled by political leaders at the highest level.
They are likely to be discussing the possibility of a transition period following Brexitduring which new trade arrangements can be finalised and phased in, in order to prevent a "cliff-edge" move to the new model.
- May 2018 - English local government elections will provide the Prime Minister with her first widespread electoral test since the disastrous snap election of June 8, 2017.
- October 2018 - Mr Barnier hopes to be able to conclude withdrawal negotiations around this point, in order to allow time for them to be ratified before the end of the two-year Article 50 deadline.
- Winter/spring 2018/19 - The European Court of Justice could be asked to rule on whether the deal requires approval by each EU state. If so, it could have to be ratified by up to 38 national and regional parliaments, with any of them effectively holding a veto.
Mrs May has promised MPs a "take it or leave it" vote on whether to accept the deal or take Britain out of the EU without agreement and fall back on World Trade Organisation rules.
The Westminster vote will take place before the European Parliament debates and votes on the deal, effectively giving MEPs the final say on whether it will go ahead.
- March 29 2019: Two years after the invocation of Article 50, the UK ceases to be a member of the EU and is no longer subject to its treaties, whether or not a withdrawal agreement has been reached.
Because the exact moment of exit is midnight Brussels time, the UK is due to leave at 11pm on March 29.
This date can be extended by agreement between all member states. It is not yet clear whether the exit clock can be stopped by the UK withdrawing its Article 50 notification.
The EU insists that a trade deal can only formally be adopted after withdrawal, though the UK hopes this will happen very swiftly, with necessary negotiations completed beforeBrexit day.
If no trade deal has been reached by this point, UK-EU relations could be governed by a ''transitional arrangement'' for some months or years. Even if a trade agreement has been sealed, it could be introduced gradually during an ''implementation phase'' afterBrexit.
- June 2019: European Parliament elections will take place without the UK.