Many Americans are notoriously bad at taking all of their vacation time. Rob Whalen used to be one of them.

In less than five years working at Cisco Systems Inc., he piled up 240 hours of paid time off. That’s a month and a half Whalen could have used to rest at home or relax on a beach. Instead he chose the office, where there was just too much to do.

Shortly after leaving that job, he was talking to a friend with an equally backed-up vacation account: “Wouldn’t it be really cool,” his friend said, “if you could use some of it to buy a hotel and airfare, and then go on a big two-week vacation?”

That gave Whalen an idea. Three years later, he is pitching PTO Exchange, a company he co-founded with Todd Lucas as a way to let workers tap the cash value of their days, weeks, and months of unused time off. The Seattle-based startup, which recently signed its first employer client, lets workers trade unused, paid time off for travel or contributions to 401(k) plans and health savings accounts.

“With so much time going unused, we realized that this is a benefit that needs to be redefined,” Whalen said.

Work hard, don’t play
When it comes to time off, Americans are in a perverse situation. The U.S. is the only advanced nation that doesn’t mandate paid leave. Workers in most European countries are legally entitled to about 20 days of vacation or more each year, according to a report from the Center for Economic and Policy Research. Swedes are entitled to five weeks, while French workers get as many as 30 days.

In the U.S., by contrast, almost a third of workers get no paid sick leave, and more than a quarter don’t get any vacation time, according to the Bureau of Labor Statistics, or BLS. That’s prompted states such as California and cities including New York to require that employers provide paid sick time.

Meanwhile, those lucky enough to get time off frequently don’t use it. Less than half of U.S. workers said they took all or most of their vacation days in the past year, and only 22 percent used the bulk of their paid sick time, according to a survey (PDF) of 1,600 employed adults by Harvard University, the Robert Wood Johnson Foundation, and National Public Radio. And it’s not just the worker bees—reluctance to be away from the office often stretches into the executive suite, too.

Instead, Americans come to work sick, spreading infection and making themselves and everyone around them miserable. Or they burn themselves out mentally by not taking vacation. As a result, millions of days intended to give people much-needed time off either get forfeited or end up on company balance sheets to be cashed out by workers sometime in the future.

“There is workplace peer pressure to minimize using” time off, said Lonnie Golden, a Penn State University economics professor who studies vacation. A company might officially offer a generous vacation package, but workers know that their bosses and colleagues depend on them to be present and productive. The tendency among companies to keep staffing lean means there isn’t always enough people to cover when someone goes on vacation.