EU leaders and UK Prime Minister Theresa May had warm words for each other ahead of an informal EU leaders' summit in Salzburg, Austria. But the remarks by Donald Tusk, President of the European Council, exposed the deep chasm that remains between the two sides on certain key sticking points such as the Irish border.In a brief statement in Salzburg on Wednesday, Tusk said that while proposals hashed out by May and her Cabinet at her Chequers country retreat are a "positive evolution," the issues surrounding the Irish border and economic cooperation need to be "reworked and further negotiated."If a deal can't be reached, Britain risks crashing out of the EU at the end of March next year in a messy fashion. The Bank of England has warned that house prices would crash, businesses fret over chaos at the Channel ports and airlines worry that the agreements that keep planes in the air across Europe would fall away.But for a deal to be concluded, each side must find a way of avoiding the necessity to rebuild border posts between Northern Ireland, which will leave the EU with the rest of the UK, and the Republic of Ireland, which will remain in the EU. The removal of border infrastructure was a key part of the Good Friday Agreement that brought peace to Northern Ireland after years of sectarian strife.Tusk warned that the clock is ticking. Under the terms of Article 50, the mechanism by which member states can leave the EU, the UK must leave the bloc by March 29 next year."Today there is perhaps more hope, but there is surely less and less time," Tusk said. "Therefore, every day that is left, we must use for talks."On Tuesday the EU's chief Brexit negotiator, Michel Barnier, suggested that he was preparing to propose a new "backstop" proposal on the Irish border, attempting to "de-dramatize" discussions about what happens with Northern Ireland if Britain and the EU fail to reach an agreement on a future relationship.May addressed her soon-to-be former European colleagues at a dinner Wednesday night at the Felsenreitschule theater, where she pushed back against Barnier's proposals.In an op-ed in German's Die Welt newspaper ahead of her arrival on Wednesday, May said that any proposals for the border on the island of Ireland between Northern Ireland and the Irish republic could not be allowed to split Northern Ireland off from the rest of the United Kingdom."Neither side can demand the unacceptable of the other, such as an external customs border between different parts of the United Kingdom — which no other country would accept if they were in the same situation — or the UK seeking the rights of EU membership without the obligations," she wrote.On Thursday the rest of the leaders, without May, will meet again. But nothing official will come out of the two days in Salzburg, as it is an informal meeting. The real deals will come in October during the first formal EU leaders meeting.On Wednesday, Tusk announced a special Brexit summit in November, when he hopes a deal will be "finalized."The timetable has been set, but the route to the destination is still to be determined.