Carlos Slim’s proposal that we work a three-day week sounds crazy. But many, in 1922, thought Henry Ford crazy when he announced that his staff would work a five-day week.

Our working week seems normal to us because it is what we have always known and what everyone else does. In much, but not all, of the world, Saturdays and Sundays are days off. But if you are old enough, you can remember when it was normal for people to work on Saturday mornings too.

So could Mr Slim, the Mexican telecoms boss and the world’s second- richest man, be heralding a change in working life to match Ford’s?

He certainly could be for those he was most concerned about when he made his three-day-week statement at a business conference in Paraguay: the workers who are not ready to retire.

As Mr Slim said, it no longer makes sense for people to stop working in their fifties or sixties when they may still have up to a third of their lives ahead of them.

“People are going to have to work for more years, until they are 70 or 75, and just work three days a week — perhaps 11 hours a day,” he said.

Keeping older employees at work makes sense for societies, especially those with a diminishing number of young people who are expected to support long-living pensioners.

It also makes sense for older employees: a mix of work and leisure is what many want. “With three work days a week, we would have more time to relax; for quality of life. Having four days [off] would be very important to generate new entertainment activities and other ways of being occupied,” the 74-year- old Mr Slim said.

He appeared to be suggesting that these short-week workers earn the same as they did full-time. That is what happens at Telmex, his Mexican fixed-line phone company, where those eligible for retirement can opt to work four-day weeks on full pay.

Older workers elsewhere might prefer shorter weeks at reduced pay — and eight or nine-hour days rather than 11. Their companies might value retaining their experience while saving money on their salaries.

What about everyone else? There are those who are unemployed, or in tenuous jobs, or on zero-hours contracts, who would be delighted to have three secure, well-paid days of work a week.

Others in hospitals, supermarkets and petrol stations have to work at night, as well as on the weekends that Ford made rest days for his employees.

But a shorter week would work for many others if their companies have the imagination to agree to it.

I have managed about a dozen working parents (almost all women) on three or four-day weeks. They have, in almost all cases, been more productive and industrious than their five-day-week colleagues. They were generally more focused and better organised.

Shorter weeks don’t work in every job, but they work in more jobs than most tradition-bound managers think. Agreeing to them requires two shifts in management thinking. The first is the realisation that much of the time spent in offices is wasted anyway. (And most of the emails sent outside working hours are unnecessary. We coped before we had the technology to send them.)

Senior managerial working hours are often the least necessary of all. Henry Mintzberg, the Canadian management writer, discovered that top executives buzzed around without focusing for too long on anything.

Sir Gerry Robinson, once head of Granada, the television group, told the Financial Times: “I’ve always worked short hours. You make very sure that you do not spend time doing things that are not important.”

Second, senior executives need to understand that the best way to measure people is by the work they produce — not by how much time they spend at their desks.

Above all, managers need to grasp that people’s lives have changed. They have children, and elderly, needy parents. There are years when their children require them more and others when they need them less. At a lunch I attended in Hong Kong last month for Asian company in-house lawyers, the women spoke about how important flexible working was to advancing their careers.

People living longer, in better health, are changing working life too. Mr Slim’s idea is in tune with the times.

Published in Dawn, Economic & Business, July 28th, 2014