The UK government doesn't have a clue.
Usually, like the (in)famous Mumsnet "no", that is a complete sentence. (Or an accurate summation of the 20th century).
However, being brutally specific, it is actually a genuine statement about the state of the UK governments knowledge of the number, and status of EU citizens in the UK.
Bluntly, they have no way of knowing when any EU citizen actually came to the UK.
Which means they have absolutely no way of introducing some mythical "pre/post" date for any supposed expulsion of EU nationals. Even if they wanted.
The only way they could enforce such a system is to start collecting details now. Only it's not as simple (or cheap !) as you might think. Plus the slighly galling fact that as long as we are members of the EU, it would be unlawful.
Now the dust has started to settle on Brexit, and the grown-ups have taken over, it's becoming increasingly clear that an awful lot of what was promised is not only undesirable, it's impossible.
Here's an article which digs into some of the issues and numbers. Probably best to apologise in advance for it's constant references to "facts", and verifiable reference sources. I'm sure we'll return to Unicorn farts soon enough.