In November 1955 The Economist ran an unsigned essay that opened with a sentence people are still quoting seventy years later: work expands so as to fill the time available for its completion.
The author was a British naval historian named C. Northcote Parkinson, and he was being funny. The essay is a piece of satire about bureaucracy, built on the observation that between 1914 and 1928 the Royal Navy managed to lose about two-thirds of its warships and a substantial share of its sailors while simultaneously growing its number of dockyard officials and Admiralty staff by rather a lot. Fewer ships. More admirals. Every year, reliably, regardless of whether there was anything for anyone to do.
Parkinson's explanation was that officials multiply subordinates rather than rivals, and that officials make work for each other, and that between those two habits an organization can grow indefinitely without anything useful happening anywhere inside it. He worked out a formula. The formula was also a joke. The formula got cited in management textbooks for decades.
The line outlived the joke
The opening line escaped the essay almost immediately and became one of those things everyone knows, and it's stuck around because it turns out to be true about roughly everything, not just the Admiralty.
Give yourself a week to write a two-page report and it will take a week. It won't be a better report. There will just be more Tuesday in it.
Anyone who has ever done a semester's reading in the forty hours before an exam has run this experiment personally and knows the finding holds. The work didn't get smaller under pressure. It was that size the whole time, and the other eleven weeks were the expansion.
Why nobody fixes it
The obvious response is to give yourself less time, which almost nobody does, because you'd have to know how long the thing actually takes, and people are famously terrible at that.
Not slightly terrible. Systematically, directionally terrible, in the same direction, every time — we underestimate how long things take and we've been doing it long enough that psychologists gave it a name and have been writing papers about it since the seventies.
Which puts you in an unfortunate spot. Work expands to fill available time, so you should constrain the time. But you can't judge the time, so your constraint is a guess, and a guess you invented is a constraint you'll happily renegotiate at 4pm.
Where the edge has to come from
The constraint has to come from somewhere other than your own optimism in the moment.
IHC estimates how long each thing will take, adds it up against the hours you actually have that day, and hands you a plan that fits. When your six hours of work don't fit in your four hours of day, it says so at the start rather than letting you find out at the end. And when you finish something in eight minutes that it called twenty, it remembers, and next time it guesses closer.
Parkinson was writing about departments that grow because nothing stops them. The modern version is a to-do list that grows because nothing stops it either, and the ambient guilt of never reaching the bottom of a list that was never designed to have a bottom.
Give the work a real edge and it stops expanding. Ours is the number of hours you've got, which has the advantage of being true.
Sources: C. Northcote Parkinson, "Parkinson's Law," The Economist, 19 November 1955.