Throughout the Brexit process, people have used the image of a ticking clock. On 29 March, unless something changes, the clock stops.
That's why political pressure in the United Kingdom to extend Article 50 (the two year negotiating period for leaving the EU - which runs out on that date) is growing.
Article 50 can't be paused. It can only be extended or revoked altogether and if neither of these things happen, the UK leaves, deal or no deal.
So, people are worried that time is about to run out.
Extending Article 50 is not government policy.
But asked by the BBC on Wednesday if there was still time to leave the EU with a deal passed through Parliament, the Justice Secretary, David Gauke, said: "It's still possible - but it is very tight. We don't have a lot of spare time, frankly."
"I think we've got days before we will start to run out of parliamentary time. I hope we can meet that timing."
These may be the words of a government minister keeping up the pressure on Parliament to agree on something.
But it's not just about agreeing a deal - although there is, as yet, no clear sign of a majority in Parliament for any form of Brexit or for an election or another referendum.
It's also that even if Parliament were to vote in favour of the Prime Minister's deal, or something like it, there is a huge amount of legislative work to do.
There are now fewer than 40 legislative days left before Brexit is due to happen. And if the withdrawal agreement were to be approved, it would then have to be turned into new legislation.
The passage of that legislation could be contentious.
The next Brexit vote - on what Theresa May is proposing to do next - is scheduled for 29 January. After that there will be fewer than 30 sitting days in which to get new laws through Parliament.
A little more time could be found if Parliament were to sit on Fridays and a planned parliamentary recess next month was abandoned.
That might buy a maximum of 14 extra days but the schedule would still be extremely challenging.
There's also a huge amount of secondary legislation that needs to be passed, making small changes to the law (sometimes just tweaking language, sometimes more complicated) via what are known as statutory instruments (SIs).
The government estimates it will need nearly 600 SIs before Brexit day.
The latest research from the Hansard Society shows that 305 SIs related to Brexit have been introduced so far - and only 67 have completed their passage through Parliament.
There are also six other pieces of primary legislation going through Parliament at the moment (on trade, agriculture, fisheries, immigration, international health care and financial services) - and all of them would need to be approved before the end of March if the UK leaves the EU with no deal.
The Institute for Government is keeping track of the legislation here.
If there was a post-Brexit transition period, during which all the rules would stay the same, there would be more time to complete the passage of this legislation (and the financial services bill would fall away completely).
But as long as a no-deal Brexit is on the table, ministers and officials have to plan for the possibility of having to pass them all by 29 March.
In exceptional circumstances, it is possible in theory to ram legislation through Parliament extremely quickly.
The 2017 Northern Ireland Budget Act, for example, passed the House of Commons in one day, the House of Lords in one day, and received royal assent two days later.
But it would be unprecedented to operate in that way, with so little scrutiny, for legislation as momentous as the EU Withdrawal Act, and it would obviously require the cooperation of both Houses of Parliament.
So what's the process for extending Article 50?
First of all, the government would need the formal agreement of all 27 other EU countries.
The EU would need to be convinced that there were good reasons for an extension, beyond prolonging unresolved political arguments in Westminster.
That could mean developments in UK politics that would change the nature of the argument over Brexit - such as a decision to call an election or another referendum.
Or, if a deal was agreed late in the day, a short extension could be considered to give a bit more time to complete the ratification process and pass the necessary legislation.
If other EU countries agreed, the government would still need parliamentary approval in Westminster for extending Article 50 via a resolution and it would need to change the date of exit in EU law.
Again, that could be done relatively easily with the support of Parliament.
But how much extra time could be needed?
Elections for the next European Parliament are being held in May and there are currently no plans for the UK to take part.
But new members (MEPs) won't be sworn in until the first week of July, which creates a three-month window.
"An extension of Article 50 until 1 July is feasible without needing to hold European parliamentary elections in the UK in May," said the Labour MEP Richard Corbett.
In theory the outgoing European Parliament could be convened in an extraordinary session in June, after the elections, to complete its part in the ratification of any Brexit withdrawal deal.
There are already calls for a much longer extension of Article 50 - but if there have been no European elections in the UK, that takes you into very tricky legal territory.
The UK would still be an EU member state but it would have no newly elected MEPs.
A confidential legal opinion, first drafted by the European parliament's legal service in 2017 and seen by the BBC, concludes that the UK would technically be in breach of its treaty obligations if it did not hold European elections while it remained an EU member state.
But - importantly - the opinion also concludes that such a move wouldn't make the new parliament illegal.
"The possibility for the European parliament to be validly constituted following the 2019 elections," it says, "would not be affected by a potential failure by the UK to organise elections."
Lawyers will continue to argue over the details, and there will be many political reservations.
Like with so many parts of the Brexit process, this is uncharted territory. And no-one is sure what is likely to happen next.