At a guess, that's the deadline beyond which their own operation is seriously hitting the buffers. They need to get Ukraine to agree to being beaten because there's no way they can sustain the pressure and actually beat them (by fair means or even most foul ones) otherwise.
If I were President Z, and if I could possibly put aside all the overwhelming humanitarian pressures being put upon me, I think I'd take this as a cue to keep on doing what I'm currently doing. Really, his people have done what I think many would have considered impossible. At great cost, but probably no greater than what they'd have to have paid if they'd just rolled-over... The only thing worse is to have paid that cost and then have to pay again by backing away and pulling any further punches.
I'm sent back to the Red Storm Rising playbook. There was a Russian plan that was 'sure' of victory, but it plainly hasn't survived contact with the enemy.
This is, of course, me totally in the position as Armchair War-Leader. I'm not the real thing, and I have no ambition to ever be put in the position of being the real thing. Those playing this particular high-stakes poker game are welcome to it, and I don't envy them playing it.
(And if they even take the actual advice of a rando on a relatively obscure forum, they're not the people I think they are. If it happens that we're of the same mind, that's just coincidence.)