On 22 March 2019 the UK and the EU agreed to defer the "exit day" for Brexit to 11pm (UK time) on 22 May 2019 if the House of Commons approves the Withdrawal Agreement before 11pm ( UK time) on 29 March 2019 but, if no such approval has occurred , then the "exit date" will only be deferred to 11pm ( UK time) on 12 April 2019.
On 25 March 2019 , the UK Government laid before the House of Commons a draft Statutory Instrument ("SI") under the so-called affirmative procedure authorising the new definition of "exit day" as a matter of UK domestic law.
On the same date, the so-called UK Department for Exiting the European Union ( DEXIT) published a briefing note explaining the confusion that would be caused if the House of Commons failed to approve the new definition of "exit day" in time. There would be a confusion between international law under which the new definition of "exit day" has already been accepted ( see Article 1 of European Council Decision (EU) 2019/476 of 22 March 2019) and UK domestic law under which no such change of date would have been approved.
As the DEXIT briefing note explains:-
"...A large volume of EU exit legislation preparing the domestic statute book for the moment EU law ceases to apply is due to enter into force automatically on "exit day". A rejection of the SI would therefore create a clash in domestic law ; contradictory provisions would apply both EU rules and new UK rules simultaneously, and in some cases new UK rules would replace EU rules prematurely...."
This is a challenge which, if it ever came to pass, would only make confusion worse confounded.
Most people would hope that there is still sufficient goodwill left in this House of Commons to ensure that this possibility does not come to pass.