LONDON — There's a curious omission from the U.K. government’s European Union (Withdrawal) Bill: a date.

Throughout the 66-page bill, which converts all EU law into U.K. law while at the same time repealing the legislation that took Britain into the bloc in 1972, the date of withdrawal is referred to simply as “exit day.”

“Exit day” is mentioned 128 times in the bill — but not once is the date March 29, 2019 set down in writing. That is the end date of the two-year Article 50 process that Theresa May set in motion earlier this year, and barring an unlikely sequence of events, the day on which the U.K. will no longer be a member of the bloc.

The omission of a date has raised eyebrows in Westminster among both Euroskeptics smelling betrayal and soft Brexiteers in the Labour Party who fear it gives the government too much power to mess with the transition out of the EU.

In the bill, exit day is defined simply as the day designated as such by the government, or “where a Minister of the Crown appoints a time as well as a day as exit day.”

The process is clarified later in the bill: Exit day will be set down in legislation as a so-called statutory instrument, which does not need a full vote in the House of Commons but in a smaller committee in which the government has a majority.

Critics of the government are beginning to wonder aloud why ministers are so reluctant to name a date — after all it's there in black-and-white in European law: Two years after the Article 50 withdrawal notice is handed in.

Unless there is a unanimous agreement to extend the negotiating process, Britain will leave the European Union on March 29, 2019, whether it is written in the bill or not.

Or will it?

Here are four reasons why the lack of a date matters — and one why it probably does not.

Brexit abandoned

Public mood can shift, often dramatically and at short notice — just look at June's general election.

If there's no deal in 2019 and a growing demand from the country not to leave without an agreement, Britain could — theoretically at least — try to withdraw its Article 50 notification.

Exit day means leaving the European Union, and it also means leaving the institutions of the EU.

The legal question of whether this is possible has not been settled — no country had triggered Article 50 before, let alone rescinded it. It’s possible that the EU27 could refuse to accept the withdrawal, at which point the case would be heard by the European Court of Justice, the EU's top court. According to one former government lawyer who spoke to POLITICO on condition of anonymity, if the EU27 unanimously accepted the withdrawal of the Article 50 notification then Britain could remain in the EU.

At this point, the fact that there is no date in the European Union (Withdrawal) Bill means the measures in what would by then be an act of parliament would not have come into force. The law would be in legal limbo, on the statute book ready to be triggered at any moment by the government while Britain remains in the EU.

Brexit delayed

Exit day means leaving the European Union, and it also means leaving the institutions of the EU. But staying within one or more of them may be a necessary condition of any “soft” transition period. Chief among those is the European Court of Justice — the arbiter of trade disputes in Europe.

Shadow Brexit Secretary Keir Starmer is particularly exercised about this point. He told MPs last week that the exact date of exit day was “crucially important” because it will be the day the European Court of Justice is “extinguished” in U.K. law.

Without the ECJ, Starmer said, the U.K. might not be able to achieve a transitional deal on the terms it is looking for.

“If exit day is in March 2019, it is difficult to see how we could transition on terms similar to those we are now on,” Starmer said. “Control over exit day is therefore hugely important. Who will have that control? This will be in the sole power of a minister.”

Otherwise, it might be necessary to repeal the EU (Withdrawal) Bill and bring it back in a different form.

Brexit stages

There could be multiple exit days, according to Starmer and the former government lawyer.

In the Commons, Starmer raised this point directly. “Members might think I am joking, but someone who drafted the bill has thought of that, and it is conceivable that there could be multiple exit days, all chosen by a minister and not by parliament.”

In other words, Britain could leave the European Court of Justice at a different date to other elements of the European Union. It would be Brexit by stages.

This would be a purely domestic matter — Britain would no longer be a member of the European Union, but it would be accepting the authority of some of its institutions.

Brexit power grab

If the bill is passed in its current form, it will be up to the government to decide when European Union law ceases to apply in the U.K. “Anyone simply passing this bill must be prepared to be a spectator on the question of what the transitional measures should be and how they operate,” Starmer said.

Brexit is a train that has left the station and the destination is set. Unless there is a dramatic change of political reality, the train cannot be stopped.

His argument is that by putting the exit date in the hands of ministers, it effectively also gives them the power to decide the point at which ECJ jurisdiction no longer applies in the U.K. That is significant because the ECJ's power over the U.K. could be crucial in any transitional deal. Critics argue that by removing the date from parliamentary scrutiny, the government is preventing MPs from having full scrutiny over that too.

The House of Lords select committee on the constitution raised similar concerns. “We are concerned that the power to define 'exit day' — a matter that is pivotal to the operation of the bill — is unduly broad in its scope and flexibility, and that it is not subject to any parliamentary scrutiny procedure.”

In other words — the bill takes back control and gives it to the government.

Calm down, it’s an insurance measure

Forget all of the above, insist government ministers and insiders when asked what is going on. It is simply an insurance against the unforeseen. Brexit is a train that has left the station and the destination is set. Unless there is a dramatic change of political reality, the train cannot be stopped.

What government lawyers and ministers have in mind, above all, is a last-minute delay in the negotiations, which will mean a little bit more time is needed.

A potentially more serious issue would be if an exit agreement — particularly the transitional deal — were resisted by one of the regional bodies in the EU over claims that it is a treaty and needs to be ratified just like a free-trade agreement.

This occurred in 2016, when Belgium's Walloon parliament rejected the EU's trade deal with Canada and threatened to kill it. In this nightmare scenario, the U.K.-EU deal could be held up in the European Court of Justice beyond March 29, 2019.