A key part of the Withdrawal Agreement between the UK and EU is the so-called Irish Backstop, the guarantee that Northern Ireland will remain in a customs union with the EU until both sides agree on alternative arrangements that will keep the border open and free of physical infrastructure.
This is deeply unpopular in the Westminster Parliament and the main reason that the Withdrawal Agreement could not be passed. A counter proposal is to add a time limit to the backstop, but the EU has rejected this.
Is there any point to having a backstop with a time limit, from the EU's point of view? I can't see any benefit at all.