By Kevin Connolly
BBC News, Washington
Brace yourself for two sorts of column heading your way from out of Washington in the coming days.
Presidents Johnson and Kennedy both achieved a lot in their first 100 days
First will come the national report cards assessing the performance of President Barack Obama - his handling of the recession, his first appearances on the global stage and the wars in Iraq and Afghanistan.
Shortly afterwards, or even in parallel, will come the articles explaining how the 100 days benchmark is absurd, unreasonable and impossible to take seriously.
Often the two streams of thought will come from the same organisations - highlighting America's ambivalent attitudes towards the first rendezvous with judgement which each commander-in-chief traditionally faces.
Bay of Pigs
News organisations of course like the whole idea of the 100 days - as we tend to like anything which allows us to use dates and deadlines to make the news agenda easier to plan and manage.
Presidents themselves have tended to dislike and distrust the idea of being held to judgement so early in their administration.
How can we judge Barack Obama's performance in the field of public health, for example, when he has not even got around to appointing a surgeon-general yet?
President John F Kennedy even devoted a few lines of his inaugural address to an attempt to forestall any critics who were planning to draw conclusions from his first three months or so in the White House - an interesting example of Mr Kennedy's obsession with manipulating the media.
He tried to make the sure the parameters of judgement were thrown as wide as possible too - warning that what he wanted to accomplish might not be achieved "within a hundred days, a thousand days, or even during our lifetimes on this earth".
It did not do him any good of course.
When we remember President Kennedy, we recall the doomed glamour and the efforts on civil rights - but we also remember the disastrous Bay of Pigs invasion of Cuba by US-trained Cuban exiles which came within his 100 days and helped shape America's view of his competence.
In American politics, the concept of the 100 days dates back directly to Franklin Delano Roosevelt, the Democrat who took over in the depths of the Great Depression and eventually led his country not just back to prosperity but to victory in the Second World War and global dominance too.
He called Congress into emergency session and pushed through a string of 15 major pieces of legislation to deal with rising unemployment and a paralysis of the financial system caused by the greed and folly of the nation's bankers (sounding familiar?).
When that first burst of legislative energy had been expended, it is said, FDR noticed that it had taken exactly 100 days.
Those measures included a number of sure-fire crowd-pleasers, like the legalisation of beer sales of course, and they have set a kind of gold standard of confident, rapid political action against which every subsequent president has been judged.
Interestingly, by the way, the actual concept of the 100 days is both much older and much less auspicious than Americans might think.
It dates back to Napoleon and was first used to describe the period between his triumphant return to France from exile and his eventual, final, crushing defeat at Waterloo - a textbook reminder from history that having something start well is no guarantee that it will finish well.
I do not think it is really clear whether or not FDR knew the historical resonance of the phrase (although he knew Europe pretty well and enjoyed reading history) but I would say it is a safe bet that more people now associate the phrase with his presidency than with the Corsican corporal and the early 19th Century.
George W Bush 'didn't have a mandate for the things he went on to do'
The historian and journalist Jonathan Alter has written a book about FDR's first 100 days and is preparing another about Barack Obama's first year in office.
Indeed, sales of that first book received a substantial boost when Mr Obama mentioned in a television interview that he was reading it - the kind of publicity most authors can only dream of.
Mr Alter agrees that most presidents have resented being held up to judgement after just three months in office - but makes the point that some American leaders really did achieve a lot.
He points, for example, to Lyndon Johnson, who passed legislation creating proper voting rights for black Americans, establishing healthcare for the elderly and expanding non-European immigration - all measures that changed the face of America.
"Even though the Vietnam War eventually wrecked the Johnson presidency, the accomplishments of his first 100 days live on," Mr Alter says.
There seems to be surprisingly broad agreement about which Presidents started well and which did not.
You cannot get beyond the Bay of Pigs with President Kennedy, for example, and Ronald Reagan, even if you thinks his economic policies created disastrous deficits, was unarguably effective in setting his own tone on tax and spending issues.
Who had the worst first 100 days?
Well, according to the influential political scientist James Thurber, you need to look no further back than George W Bush, who won the presidency in the Supreme Court after effectively tying the election itself with Al Gore.
Mr Thurber argues: "He didn't read the election very well. He won by one vote in the Supreme Court - that's a mandate for moderation. He didn't have a mandate for the things he went on to do."
Of course even in the media, there is a degree of acknowledgement that the judgement at the 100-day mark is interim and transitory by its very nature.
Commentators tend to focus on the tone of the new leader and on his ability to get things done on Capitol Hill.
Take Mr Obama for example.
He has demonstrated that he can get his stimulus package passed ($789bn) and a budget with a huge deficit too ($1.75tn).
So he scores highly for effectiveness, but if those policies leave the economy hobbled with debt and fail to produce growth then that achievement will look very different somewhere down the road.
So we shall see, but for the moment we should take counsel from Napoleon, whose story reminds us so effectively that history is all about how you finish, not how you start.