Sue Ellen King had circled her retirement date on the calendar: March 8, 2015.

She had worked as a critical care nurse and nursing educator at University of Florida Health in Jacksonville, Fla., for 38 years; co-workers joked that she was there when the hospital's foundation was laid, which happened to be true.

So the send-offs went on for days — parties in the units where she had worked, a dinner in her honor, gifts including a framed photo signed by colleagues.

King felt ready. She had turned 66, her full Social Security retirement age. She had invested fully in the hospital's 401(k) plan and consulted with a financial adviser.

She and her husband, who had already retired, had paid off the mortgage on their three-bedroom ranch. They took a week's trip to Hilton Head, S.C., to celebrate their impending freedom. But her retirement lasted just three months.

"I'd done all the preparation, except to really think about what life was going to be like," King said. Days spent organizing recipes and photos, and lunching with friends, proved less engaging than expected.

So when her hand-picked replacement needed a maternity leave, King jumped at the chance to return for three months.

Now back at work in a part-time position she designed for herself, she calls herself "a failed retiree."

Economists refer to this sort of U-turn as "unretirement." (In "partial retirement," another variant, an employee cuts back to part-time status but doesn't actually leave the workplace.)

Unretirement is becoming more common, researchers report. A 2010 analysis by Nicole Maestas, an economist at Harvard Medical School, found that more than a quarter of retirees later resumed working.

A more recent survey, from RAND Corp., the nonprofit research firm, published in 2017, found almost 40 percent of workers over 65 had previously, at some point, retired.

"We definitely see evidence that retirement is fluid," said Kathleen Mullen, a RAND senior economist and co-author of its American Working Conditions Survey. "There's less of the traditional schedule: work to a certain age, retire, see the world. We see people lengthening their careers."

A Pew Research Center analysis of data from the Bureau of Labor Statistics supports that observation.

It reported that the proportion of Americans over age 65 who were employed, full time or part time, had climbed steadily from 12.8 percent in 2000 to 18.8 percent in 2016. More than half were working full time.

Even more people might resume working if they could find attractive options. "We asked people over 50 who weren't working, or looking for a job, whether they'd return if the right opportunity came along," Mullen said. "About half said yes."

Why go back to work? We hear endless warnings about Americans having failed to save enough, and the need for income does motivate some returning workers.

But Maestas, using longitudinal data from the national Health and Retirement Study, has found that the decision to resume working doesn't usually stem from unexpected financial problems or health expenses.

"It looks like something people are doing intentionally, instead of an oh-my-God response: 'I'm running out of money; I have to go back to work,' " she said. "It's much more about a choice."

Longer lives, better health and less physically taxing jobs than in previous generations help provide that choice, Maestas pointed out.

"You hear certain themes: A sense of purpose.

Using your brain," she said. "And another key component is social engagement." Earning money, while welcomed, rarely proved the primary incentive.

Michelle Wallace, who lives in Broomfield, Colo., learned about purpose over 20 frustrating months.

After decades in telecommunications, she said, she retired abruptly from a project management position in 2013, when her workplace turned chaotic and hostile.

She had saved enough to feel economically secure. But without a job, "I felt like I was free-floating, bobbing along on the ocean," she said. "I felt very ungrounded." Friends noticed her becoming more reclusive; her doctor increased her antidepressants.

In 2015, she took a part-time job with a small business that supports government researchers. Now 69, she has no interest in retiring again. "As long as somebody wants me, I have a lot to contribute," she said.