I recently came across a strange inconsistency regarding Samuel Johnson's A Dictionary of the English Language: while every (online) source I've looked at agrees that it was first published in 1755, they don't appear to agree on how much time it took Johnson to finish compiling it.
For starters, the Wikipedia article for the dictionary itself seems to contradict itself on this matter. The opening paragraph states (emphasis mine):
Johnson took seven years to complete the work, although he had claimed he could finish it in three.
But later on in the article in the section Johnson's preparation is the following claim (emphasis mine):
Johnson's dictionary was prepared at 17 Gough Square, London, an eclectic household, between the years of 1746 and 1755. By 1747 Johnson had written his Plan of a Dictionary of the English Language, which spelled out his intentions and proposed methodology for preparing his document.
This would imply that it actually took him eight or nine years to complete, depending on whether you count writing the Plan as working on the dictionary itself. And in fact, on Johnson's main Wikipedia page, the listed number is eight (emphasis mine):
In comparison, the Académie Française had 40 scholars spending 40 years to complete their dictionary, which prompted Johnson to claim, "This is the proportion. Let me see; forty times forty is sixteen hundred. As three to sixteen hundred, so is the proportion of an Englishman to a Frenchman." Although he did not succeed in completing the work in three years, he did manage to finish it in eight.
The British Library's website agrees that it took "just over eight years" for the dictionary to be finished. Unfortunately, nearly every other source I've found says that it actually took nine years, including this info page for an online version of the dictionary (emphasis mine):
More than once he discovered that his procedures weren’t working, and had to toss out his work and start over again. It is no surprise that he missed his three-year deadline. In fact it is almost miraculous that he completed the work in nine years, from the signing of the contract in 1746 to publication in 1755.
Assuming that Wikipedia is correct that Johnson signed the contract to write the dictionary in June 1746 and that the final product was published in April 1755, that would mean a total working time of a few months shy of nine years, which I certainly wouldn't call "just over eight years."
So... what's going on here? Is this just an issue of semantics or a rounding problem? Is one or more of these sources incorrect? What is the definitive correct answer to the question of how long it took Johnson to complete his dictionary?