The PM wants a general election on 15 October.
Tory rebels, led by Sir Oliver Letwin, and many Labour MPs, including frontbenchers, want polling day to be any time after 31 October.
What is this dispute all about? It is not about whether an election is coming. After yesterday's Tory defection and mass expulsions - what one of those exiled, Sir Alistair Burt, calls a purge - Boris Johnson no longer has the numbers to govern in any meaningful sense. Paying the wages of Johnson and his team in these circumstances would be the very height of fiscal waste.
Also, all MPs accept - the Tory rebels and opposition parties with glee, Boris Johnson and most Tories with dismay - that the backbenchers' bill being debated now, which would force the PM to ask for a Brexit delay, will become enacted at the end of this week.
Shenanigans over dissolution dates and election timing are no longer about warding off a new law compelling Johnson or another PM to sue EU leaders to postpone Brexit.
But where Johnson disagrees with Letwin and the likes of Labour's Sir Keir Starmer is that he wants the election to be timed early enough - on 15 October - so that it could be repealed BEFORE the 17 October EU council.
And his opponents want polling day in November or later, so that Johnson would in theory have failed to have honoured his pledge to take the UK out of the EU on the due date.
The point is that the rebels and opposition parties are salivating at the notion that Johnson could be humiliated by being forced under the terms of the legislation to ask EU leaders on 17 October to move Brexit day from 31 October to 31 January. They expect and hope that if Johnson becomes their puppet in Brussels, he would then be flattened under Nigel Farage's steamroller.
But in practice he is much more likely to be compelled in those circumstances - for reasons of self and party preservation - to out-Farage Farage, and transform the Tory Party itself into the no-deal Brexit party, inflexibly opposed to any kind of Brexit deal.
The point is that whether the election happens before or after the end of October, it will be fought between a relatively "pure" Brexit Tory party and a series of opposition parties all opposing Brexit to a greater or lesser degree (Lib Dems and SNP wanting no Brexit, Labour urging a referendum).
The longer the delay to polling day, the more extreme will be the Brexit position taken by Johnson's Tory party.
Which feels precisely the opposite of what many - though not all - of Johnson's opponents would want. Because he could yet win an election and push through the most abrupt rupture with the EU, which his critics profess to detest.
And in the course of that election, we'll witness - under the stewardship of Johnson's chief aide, Dominic Cummings - a more remorseless and relentless attack on those who support remaining in the EU than anything we might be capable of imagining.
If you worry for our ability to heal as a nation, this should be a concern.
As one member of the government put it to me when I asked how the day had been, he said "it's like smashing a drawer against your testicles multiple times, but without the adrenaline rush".