Joseph Coughlin, head of the MIT AgeLab, notes that retirees at 65 still have one-fourth of their life to live. (Longevity Economy/The Minnesota Star Tribune)

Here's a stunning number: Retire from a full-time job and suddenly you have some 2,500 hours available to you each year that previously went toward your work.

"People enter retirement at different ages and with various levels of resources. But all new retirees are time rich," writes executive coach Joe Casey in his new book, "Win the Retirement Game: How to Outsmart the 9 Forces Trying to Steal Your Joy."

"The question is: How will you invest that time?" he adds.

Casey's number reminds me of a striking calculation from Joseph Coughlin, director of the MIT AgeLab. Coughlin notes the odds that a person with some or more college education and a good income will live well into their 80s are high.

He divides life into four periods averaging approximately 8,000 days each. Birth to college graduation at age 22, about 8,000 days. College graduation to midlife at age 44, another 8,000 days. Midlife to retirement at age 66, yes, 8,000 more days.

"And, add about another 8,000 days from your retirement party forward," he writes.

Time and money are often put together, with good reason. A classic example is the power of compound interest over time. Supposedly Albert Einstein called compound interest the eighth wonder of the world.

"He who understands it earns it; he who doesn't pays it," he allegedly said.

The power of compound interest lies behind the personal finance mantra to start saving early for retirement.

But there is another aspect of time and retirement that doesn't get enough emphasis. How will you take advantage of the annual 2,500 hours or the approximately 8,000 days available to you in retirement? What will bring you purpose and meaning, social connections and fun in the next stage of life?

How to invest your time is a huge topic. But there's an idea in Casey's book I like: Building a multipurpose retirement.

Some people find a passion, a calling that dominates their time in retirement. Most of us have a portfolio of interests that give both purpose and pleasure.

The portfolio these days often includes some work that brings in an income and relieves the stress many people feel about running out of savings in their elder years.

The pursuit of a multipurpose retirement involves a combination of deliberate planning and a willingness to experiment to find the right mix for you. The effort is worth your time.

Farrell is economics contributor to the Star Tribune, Minnesota Public Radio and American Public Media's "Marketplace."