(CNN) -- There's much that's still unknown about Brexit as Britain concludes its sixth round of talks with Europe, but what has been revealed is the exact hour of Britain's departure.
In an opinion piece published in Britain's Daily Telegraph newspaper on Friday UK Prime Minister Theresa May said Brexit would formally take place at 11 p.m. GMT on March 29, 2019.
She also warned that she would "not tolerate" any attempts to delay this eleventh-hour departure -- which will fall at midnight in most European capitals.
"Let no-one doubt our determination or question our resolve, Brexit is happening," May wrote.
The comments come at the end of a turbulent spell for May's government, which saw the second resignation of a Cabinet minister in a week, a serious gaffe by her top diplomat, further twists in the Westminster sexual harassment scandal and renewed questions about her leadership.
On top of that, business and EU figures have warned that the clock is running down on Britain's chances of making "sufficient progress" on the terms of its exit for the European Union to agree to start talks on a transition period and future relations -- including, crucially, trade -- after a crunch EU leaders' summit next month.
Britain's Brexit secretary, David Davis, and the EU's chief negotiator, Michel Barnier, ended the sixth round of talks Friday in Brussels, Belgium, with talk of some progress on the three key elements of the UK's exit -- what it will pay, the Northern Ireland border, and citizens' rights. But as expected no major breakthrough was announced with all eyes on next month's summit.
Most significantly, Barnier told reporters it was "absolutely vital" to deal with the so-called divorce bill within the next couple of weeks if the UK wants to begin talks on its future trade relationship with the bloc.
Here's what we still don't know about Brexit:
How much will Britain pay?
The stickiest issue is the "divorce bill," or what the UK will pay to settle its financial obligations to the bloc -- although Barnier said Friday he wanted to see "sincere and real progress" on all three elements before moving forward.
Speaking in Florence in September, May said she would honor financial commitments the UK made while an EU member, but EU leaders complain that her words have not translated into a specific monetary offer from British negotiators.
May has also insisted that there can be no final agreement on the financial settlement until there's also agreement on the future trading relationship between the EU and UK. Different sums have been bandied about over the past few months, from 20 billion to 100 billion euros.
Assessing the outcome of the sixth round of talks on Friday, Barnier said: "On the financial settlement, we need to work over the next few weeks on the objective interpretation of the undertakings entered into by Prime Minister May in Florence. I think this is absolutely vital if we are to achieve sufficient progress in December."
He added: "It is just a matter of settling accounts, as in any separation."
Asked by a reporter if he would need "clarifications or concessions" from the UK within the next two weeks for talks to enter the second phase, on future relations, in December, Barnier said, "My answer is, yes."
Davis, meanwhile, said that "substantial technical progress" had been made on financial settlement issues and reiterated that the UK would "honor the commitments we have made during the period of our membership."
The fifth round of talks ended last month in apparent deadlock, with Barnier saying discussions with Davis had failed to produce progress on the key issue of how much Britain should pay to leave.
May faces pressure from within her government -- and from large portions of the UK electorate -- not to offer too much to settle Britain's account. But at the same time, the calls from UK and international business leaders for greater certainty as the clock ticks down toward March 2019 may force her to give ground in order to move on to discussion of future relations and trade.
Where will citizens' rights stand?
Another stumbling block is what happens to UK citizens living elsewhere in the European Union and EU citizens living in Britain after Brexit.
May told the UK House of Commons last month that Britain and the EU were united on the key principles of EU citizens' rights, and were within "touching distance of a deal."
It's a complicated issue with potentially far-reaching consequences. Roughly 3 million people from other EU countries live in the United Kingdom, while around 1.2 million Britons reside elsewhere in the European Union.
The UK published new detail Tuesday on a "streamlined, low-cost and user-friendly" process for those EU citizens seeking to gain "settled status" in Britain after Brexit, saying it wanted to offer them reassurance.
Barnier said Friday that the sides were "making a fair degree of progress," including clarification from the UK on how the process will work and how citizens can appeal if their applications for settled status are unsuccessful. But, he said, there are "still a number of points that need more work," including on family reunification, people's right to "export" social security benefits and the role of the European Court of Justice.
Davis said the two sides had continued to make progress on citizens' rights and "are now seeking political solutions to the last outstanding issues on both sides." It is a "key priority" for the United Kingdom to retain the sovereignty of its courts, Davis said.
What will happen in Northern Ireland?
Both sides agree that they want to avoid a "hard border" between the Republic of Ireland, which will remain part of the European Union after Brexit, and Northern Ireland, which is part of the United Kingdom.
There is free movement across the border and many businesses have facilities on both sides.
A key question is how to avoid the need for border checks if Northern Ireland, as part of the United Kingdom, is no longer part of the EU single market and customs union after Brexit.
Free movement across the border was a key part of the Good Friday Agreement, the 1998 accord that helped bring peace to Northern Ireland after decades of sectarian conflict.
Davis said Friday that the two sides had had "frank discussions about some of the big challenges around the border" and that the UK was firmly committed to to avoiding any physical infrastructure there.
"We respect the European Union desire to protect the legal order of the single market and Customs Union. But that cannot come at cost to the constitutional and economic integrity of the United Kingdom," Davis said. "We recognize the need for specific solutions for the unique circumstances of Northern Ireland. But let me be clear. This cannot amount to creating a new border inside our United Kingdom."
Barnier said the two sides had common goals. "The unique situation of the island of Ireland requires specific solutions," he said.
The 'Brexit Bill'
May, who heads a minority Conservative government since a disastrous snap election in June, faces a battle to get the EU Withdrawal Bill, known as the Brexit Bill, through Parliament.
She's announced her intention to put forward an amendment adding the precise time and date of Brexit to bill before it goes before lawmakers next week for further scrutiny. But what's not yet known is what impact the many amendments put forward by other lawmakers will have on the legislation.
May wrote in her opinion piece Friday that the government would listen to any proposed amendment to improve the bill. "But I am just as clear of this: we will not tolerate attempts from any quarters to use the process of amendments to this Bill as a mechanism to try to block the democratic wishes of the British people by attempting to slow down or stop our departure from the European Union," she said.
Not too late to forget it all?
Meanwhile, the man credited with authoring the clause which allows a country to leave the European Union -- Article 50 -- has said it's not too late for the United Kingdom to change its mind.
May wrote to the European Council President on March 29 this year to formally notify him of Britain's "intention to withdraw from the European Union."
But Lord John Kerr argues that the letter was only a notification of the UK's "intention" to withdraw and that Article 50 has been "misrepresented" in the current political debate.
"The fact is, it is a political decision that has been made in this country to maintain there can be no going back," he told an Open Britain event in London on Friday.
"Actually, as far as Brussels is concerned, as far as the treaty is concerned, this country still has a free choice about whether to proceed. As new facts emerge, people are entitled to take a different view and there is nothing in Article 50 to stop them. And I think the British people have the right to know this, they should not be misled."
CNN's Simon Cullen contributed to this report.
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