New Conservative leader, Boris Johnson, proclaimed in his first speech as Prime Minister:
We are going to fulfill the repeated promises of parliament to the people and come out of the EU on October 31, no ifs or buts. And we will do a new deal, a better deal that will maximise the opportunities of Brexit while allowing us to develop a new and exciting partnership with the rest of Europe.
So, the question is: will Brexit - an event after which UK is no longer part of the EU - occur by the end of the day (UTC) October 31, 2019?
The question resolves positively if UK is no longer part of the European Union by the end of the day October 31, and negatively if it still remains substantially in its current status. For the purposes of the question, the cutoff point is defined as the moment when the treaties of the EU cease to apply to the UK. This can happen, for example: by withdrawal agreement between UK and EU, or by the end of the negotiating period as set out by Article 50 of the Lisbon Treaty (provided it is not extended further without any change in status happening - in this case the resolution is negative), repeal of the 1972 European Communities Act, etc. If the agreement specifies that the UK is no longer part of the EU, and is in force by October 31, the resolution is positive even if implementation of some parts of the agreement - e.g. trade deals or customs regime or visa policy - is happening past October 31.
If the UK leaves the EU but parts of the UK remain in the EU (e.g. Scotland, Northern Ireland) with some kind of special status, the resolution is still positive as long as the main part of the UK - the one inheriting the current government structure - leaves the EU.
If there is the substantial change in status, but the links with EU are not severed completely, e.g. if some special status for the whole UK is introduced which is intermediate between full membership and full withdrawal, the resolution is ambiguous.