Unless the British people change their minds, which currently seems unlikely, we will be out of the EU at the end of March 2019.
There is no dispute about that. It is about the least ambiguous element in Article 50, which provides the rules for our EU divorce talks.
Which is why Theresa May’s last-minute attempt to enshrine that leaving date in the EU Withdrawal Bill, currently being debated by MPs, is seen by many even of her colleagues as all about gesture politics and nothing about how to help the UK secure the best Brexit deal.
What the erstwhile Remainers who now think of themselves as pragmatic Brexiteers fear is that enshrining that date in UK law could actually damage Britain - by making it much more complicated to extend divorce talks in the highly probable circumstance that those talks are not concluded in the next 16 ½ months, and also by limiting the scope and richness of any transition arrangement.
Apart from anything else, May’s Brexit minister David Davis has already conceded that the prospect of everything being wrapped up much before the very last nano-second is pretty remote, if the EU’s entire history is any kind of guide.
So it is little wonder that disappointed Tory Remainers like Anna Soubry and Dominic Grieve are planning to join forces with Labour to defeat this new clause in legislation that will shape our post-Brexit future.
And as I said last night on News at Ten, such a setback for May would be devastating for her already battered authority and would inject poisoned into an already fractious Tory party.
The question therefore is why on earth she thought enshrining the formal Brexit date in British law was anything but a cheap gimmick with the potential to catch fire and burn her.