Indeed, if "a week is a long time in politics" as the late Harold Wilson, also a former British prime minister, once stated, then 21 months could be an eternity in the UK's current volatile political environment.
The first issue at the Brussels talks will be the status of millions of European Union citizens living in the United Kingdom and British residents of the other 27 countries, including their right to stay, to work, and to access medical care.
The most senior officials on either side will lead work on efforts to resolve the problem of the border between Northern Ireland and the Republic of Ireland, a situation Mr Barnier acknowledged was "politically sensitive" at a time when the Tory government was seeking the support of the Democratic Unionist Party to prop up Mrs May's minority government. "A deal like no other in history", Brexit Secretary David Davissaid in a statement as he headed into the talks.
Another scoundrel in the eyes of Eurosceptics, the European Parliament Brexit co-ordinator's role will grow as the talks near their end because MEPs will have a veto over any final deal.
A spokesman for the Department for Exiting the European Union said: "We believe that the withdrawal process can not be concluded without the future relationship also being taken into account".
Mr Barnier's insistence on sticking to the EU's priorities for the negotiations comes after Mr Davis appeared to soften his stance on the schedule for the talks.
German Chancellor Angela Merkel says she hopes for a "good agreement" after Brexit talks in which 27 European Union countries will listen carefully to what Britain wants but also defend their own interests.
"So it's the same as it was before".
But he said that Monday morning's terror attack in London and the devastating fires in Portugal reminded him that "there is more that unites us than divides us".
"It's not about punishment, it is not about revenge". And if Britons are turning against May's agenda, will European Union leaders offer them a face-saving compromise similar to that offered to Norway, which remains outside the EU's institutional structures, but accepts most of the obligations and costs of European Union membership in exchange for the commercial benefits of the single market?
"And the consequences are substantial".
"The UK will remain a committed partner and ally of our friends across the continent", Davis said.
"The pessimist see difficulty in every opportunity, the optimist sees opportunity in every difficulty".
"There is a long way to go, but we are off to a promising start".
Britain faces the hard fact that the community has more pressing matters to discuss than Britain's parting wishes.
"Now, we have a shared responsibility to deliver quick and substantive progress".
A source in May's Conservative Party said talks continued on Friday.
Whitehall believed they could negotiate a future trade deal while also working through the three strands of the divorce, namely the settling of the UK's debts with the European Union, the rights of European Union citizens living in the United Kingdom and UK citizens living in the European Union, and the Irish border.
Terms of reference agreed by both sides envisage four rounds of talks on the first phase of discussions, in the weeks starting July 17, August 28, September 18 and October 9, implying trade talks are unlikely to open until after the European Council summit of October 18/19.
Mr Barnier, who prepared for the opening of negotiations by spending the weekend in the French Alps, presented Mr Davis with a traditional walking stick from the Savoie region made from wood, hand carved and with a leather wrist strap.