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While many workers are eagerly anticipating their winter vacations, just as many have nothing on their calendars but work. In fact, employees in the U.S. leave much of their paid time off unused, according to a report by jobs and recruitment website Glassdoor. It found American workers on average use only 54 percent of their eligible vacation time. Of those employees who get paid time off, more than 90 percent have taken at least some of those days over the last 12 months, Glassdoor said. Yet only 23 percent took all of the time they were entitled to — and nearly 10 percent took no paid time off at all. The job site polled more than 2,000 adults in March and April.

The 206 million vacation days that could not be rolled over, banked or paid out last year added up to about $66.4 billion in lost benefits. For the average worker, that comes out to $604, according to a separate report by Project: Time Off, which is sponsored by the U.S. Travel Association. Katie Denis, the group's lead researcher, projects it will be more of the same for 2017. "The reality of the calendar starts to become much more pressing when you get to the last quarter," Denis said. "[But] when you think about what people are giving up, it's significant," she said. "You are essentially working as a volunteer — that's not good for the broader economy, or for individuals, either."