A dubious work perk is seeping into America’s businesses: unlimited vacation time for employees.

The policy offers some less-than-obvious advantages for employers, along with what would seem to be obvious benefits for employees.

But it could be that only the employer benefits are real, and the employee benefits are mostly imaginary.

The practice is gaining ground in places like Silicon Valley, where start-ups spring up like weeds and venture capitalists who have never seen a timecard make little if any distinction between “work” and “nonwork” time for themselves or their employees.

For employers, the benefits of unlimited vacations include not having to use staff time to track and record vacation hours. There’s no need to pay employees for their “accrued” vacation time if they leave; they don’t accrue any. Lastly, in theory at least, it’s a perk that makes a business look employee-friendly.

Among the companies that have already put an unlimited vacation policy in place are Netflix; Best Buy; Zynga, the online game maker; and Rodale, the magazine publisher.

The system could have advantages for employees caught trying to parcel out their precious vacation days, especially if the employer has a “use ’em or lose ’em” policy on vacation days at the end of the year. Did you forget to save days for that Thanksgiving or Christmas trip? No problem. Just take some more.

If you’re guessing that an unlimited vacation-days policy might have a hidden downside, you are right. Some critics have even called it a “no vacation” policy.

In a highly competitive, workaholic atmosphere, and without some guidelines as to what is normal or expected, employees may feel guilty about taking off any time at all. Are they slackers if they take two weeks? Three? Four? What’s appropriate? Will others who take less vacation, or no vacation, be viewed more favorably?

In practice, lower-level managers charged with seeing that their department’s work gets done may be left to juggle the policy. Yes, Joe’s got “unlimited” vacation – as long as it isn’t at the same time as Jill’s, Jane’s, or James’s. And if a manager does grant that six-week sojourn to join an Antarctic expedition or visit long-lost relatives in a remote part of the world, will other employees become jealous or resentful?

Workplace legal experts also point out that an unlimited vacation policy would have to work in concert with other policies on absence such as extended illness, maternity/paternity leave, or military leave, which usually have designated limits.

The United States is the only wealthy country in the world that has set no national policy on vacation days. So unlimited could truly mean anything, from many days off to none.

At least one employer has tried to modify unlimited vacations in a way that will benefit employees. Evernote, a software company in Silicon Valley, pays a $1,000 yearly bonus to employees who take at least one week off. That kind of policy might cut down on the 57 percent of US workers left with unused vacation time at the end of the year, according to a 2011 Harris Interactive survey.

In the 21st century, the workplace is evolving rapidly away from a 19th-century assembly-line model, which required a “shift” of employees on the site working shoulder to shoulder for a set period of time. Employers now offer flexible work schedules and work-at-home options, as well as other perks such as on-site child care or free snacks and beverages to keep workers happier at their desks.

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Someday totting up vacation days may seem as archaic to office workers as the sound of clacking typewriters or the sight of a uniformed attendant running the elevator.

But workers may miss doing that vacation math all the same.