It will take about three minutes to read this column. Whether it’s worth three minutes depends on me, of course. I will do my best. But it also depends on you, on your attitude to time and, perhaps, on your profession.

Twenty years ago, M Cathleen Kaveny, a professor of law and theology, began an article with the observation that “Many lawyers are very unhappy, particularly lawyers who work in big firms. They may be rich, and getting even richer, but they are also miserable, or so they say.” Was this sad state of affairs caused by long hours or stressful work? Perhaps. But Kaveny identified a more specific culprit: the “billable hour” — or even more precisely, the billable six-minute increment.

By accounting for every moment of their working lives, and defining each moment as either “billable” or, regrettably, “non-billable”, lawyers were being tugged inexorably towards an unhappy, unhealthy attitude to the way they spent their time. Not all lawyers, of course. And not only lawyers, either.

Kaveny had several concerns. She noted that lawyers would focus on narrow short-term goals rather than broader or deeper values such as maintaining skills, mentoring young colleagues, or living up to the highest ideals of the law. She worried about the explicit commodification of time.

But perhaps more relevant today than ever is that the billable hour encourages us to view all time as fungible. If time is money, that’s as true for 6am on Christmas morning as it is for 2pm on Friday the 29th of April. “No time is inherently sacred or even special,” writes Kaveny.

If you bill £1,000 an hour, as some senior lawyers do, then any particular six-minute increment of time is available to be turned into £100. Can you really afford an hour in the gym? Can you really afford to call your mother or read a bedtime story to your child?

The point is not that lawyers never call their mothers. It’s that the whole framework of the billable hour makes it feel naggingly expensive to do anything non-billable. As Oliver Burkeman laments in his excellent book Four Thousand Weeks, the logical conclusion is that “an hour not sold is automatically an hour wasted”.

If lawyers are trained to think of time in billable six-minute increments, what about other professions?

Compare the lawyer with, say, a dairy farmer. The dairy farmer works long and gruelling hours, and, typically, for far less money than the lawyer. But a large difference between the two jobs is that time isn’t fungible in the same way. The cows need to be milked when they need to be milked. And having milked them before breakfast, there is no temptation to milk them again after breakfast.

These long, non-negotiable hours can’t be easy and, just as a lawyer may be disturbed by a late-night call from a client, a dairy farmer may have to rise after midnight to help a cow in labour. But I don’t think I’m over-romanticising to suggest that just as there is something psychologically corrosive about the fact that the lawyer can always bill another six minutes, there is something psychologically healthy about the fact that the farmer can sometimes rest assured that there is nothing useful to be done until the morning.

I don’t mean to moralise, merely to observe that there is a distinction here that goes beyond earnings or hours. These different vocations have a different attitude to time baked into them.

So do others. We journalists, for example, tend to think in terms of deadlines. When I was starting out, a friend advised that the secret to journalism was “usable copy, on time”. That gets to the heart of things: journalists want to deliver agenda-setting interviews, earth-shaking scoops and tear-jerking prose, but the deadline is absolute. Everything else must strain to fit.

Deadline stress is acute rather than chronic, and so many people (myself included) find it stimulating, satisfying and healthy. Whether you’ve played a blinder or muddled through, you can file your copy and start with a clean slate in the morning. This is not such a bad attitude to work, but it runs against the grain of some careers, and with the grain of journalism.

While a journalist might watch a deadline approaching with a burst of adrenaline-fuelled productivity, for someone working fixed hours, the ticking of the clock towards 5pm might mark slow and tedious minutes that must be endured.

It is tempting to offer some typology of different professions and their attitudes to time. Yet I suspect the types are beginning to blur. In 1992, the economist Peter Sassone coined the phrase “the law of diminishing specialisation”. Thirty years later, it is astonishing how much knowledge work is handled using the same tools and workflow — a workflow that increasingly involves no fixed hours and no fixed location. We are all, like the lawyers, able to do a little bit of extra work before bedtime, even if not all of us can charge £1,000 an hour for it.

And while the “billable hour” can be a psychological trap, it does teach us one valuable lesson: there is a distinction between working and not working. It’s a distinction worth sustaining.

Tim Harford’s new book is “How to Make the World Add Up”

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​Letter in response to this column:
Avoid the trap of managing only what you can measure / From David Boorer, Llandovery, Carmarthenshire, UK