It is an arbitrary number, of course, and it offers almost no clue to the eventual verdict, either of the voters or of history. But it is still a landmark and one which any elected leader knows will be seen as setting the tone for what follows. Barack Obama's first 100 days were always bound to be subject to particular scrutiny, not only because of the hopes generated by his campaign, but because of his uniqueness as the first African-American President and the national and global economic crisis in which he took over.
Now, as so often, it is as though the two sides of the Atlantic existed in different worlds, and were judging two different Presidents. In the United States, the grumbling has already set in. Although more than two thirds of Americans believe that Mr Obama is doing a good job – far higher than for either of his two predecessors at a similar stage – the approval rating is not as positive as for either Kennedy or Eisenhower. The wave of euphoria that greeted Mr Obama's election has subsided fast.
In part, this is because the hopes he raised were so high, especially among America's disadvantaged. In part, it is because the Republicans, still licking the wounds from their defeat, have spurned the cross-party co-operation Mr Obama favoured. There is no political unity on how to tackle the economy. But it is also because the ideological fissures, so deep during both the Clinton and Bush presidencies, were reopened by Mr Obama when he published the White House documents relating to CIA interrogations. His efforts to draw a line under this shameful chapter had the opposite effect. They polarised opinion and left him flailing, uncharacteristically, between two extremes.
From abroad, the back-stabbing intricacies and self-absorption of Washington politics loom less large. Indeed, they fade into the background compared with the scale of Mr Obama's feat in winning the election in the first place and the calm assurance with which he has handled his first months.
Within days of entering the White House he had announced the closure of Guantanamo, set a timetable for the withdrawal from Iraq, rewritten Mr Bush's ban on state aid for stem cell research and prepared a bill, now passed into law, to rescue the US economy. He has also reversed the official US stance on climate change, earmarking money for renewable energy and drafting plans to regulate carbon emissions.
The new openness towards the outside world, the charm offensive in Europe, the inclusion of Turkey in the presidents first major foreign trip, and the hand outstretched to such unlikely recipients as Iran and Cuba, have inspired interest and goodwill towards the US where there was hostility and fear. It is no exaggeration to say that Mr Obama has transformed the international atmosphere. This is a foundation he now has to build on.
At home, reform of the inequitable and expensive health system is one of the few areas where he has still to make a start. But over-ambition here, as Hillary Clinton's 1993 experience showed, holds its own dangers. The appointments process has also run less smoothly than he might have hoped. But to argue, as some of his critics do, that he has moved too slowly is to misjudge the speed at which change can happen, even in the fast-paced United States. It is true that many of Mr Obama's ringing words have yet to be converted into deeds. But 100 days is but a fraction of a presidency. It would be a faint heart or a fool who would say, so soon, "No, he can't".