This is now the endgame, but it is hard at times to get across the scale, complexity and longevity of the Iran nuclear talks let alone their huge global significance, but let me try and give you a taste with a few anecdotal facts.
The road towards nuclear negotiations between Iran and global powers began when my now teenage daughter was just over two years old, the same year that an Iranian friend and fellow correspondent who is covering the talks in Vienna was also a 14-year-old teenager like my daughter.
The current US negotiating team alone (and there have been many before them) led by Secretary of State John Kerry has travelled to Vienna for nuclear talks 11 times last year and seven times this year which has been enough to celebrate the birthdays of every member of the team at least once.
The US diplomats have travelled across the Atlantic 69 times, travelling 400,000 miles - enough to cirumnavigate the globe 16 times, and it's meant that John Kerry has spent more time in a single place than any other US Secretary of State in modern times, now well into his third week in Vienna.
If nothing else, this is a an agreement that has been pursued relentlessly with iron determination by Iran and its negotiating powers. They now stand on the brink of a deal that will not only be truly historic but will lay the foundations for the most significant transformation in strategic dynamic of the Middle East since the 1979 Iranian Revolution.
The wordings of the technical agreements and the annexes which will allow for the international monitoring of Iran's nuclear programme are largely in place, the recent and most difficult rounds have tried to grapple with the toughest political obstacles; the pace and scale of the lift of international sanctions on Iran which have to be agreed to not just by the US, but also by the EU and the UN in separate votes.
There's also the thorny question of the lifting of an arms embargo on Iran.
But even after an agreement is reached, things will not happen immediately. Quite aside from the huge task of the legal teams of the Iranian and US governments having to cross-verify the English and Persian versions of the nearly 100-page document, the deal will face a fierce political battering in the US where the Republican controlled and hostile Congress will undoubtedly pick it apart, but also by hardliners in Tehran and, of course, by the Israeli and Saudi Arabian governments who have heavily lobbied against this deal.
But an agreement in Vienna in the coming hours and days will be the start of a process that could change nearly four decades of hostility and misunderstanding between Iran and the US.