If the numbers in your mutual fund investment plan aren’t adding up the way you’d hoped they would, maybe you miscalculated time.

Time is probably the most underrated, overlooked and misunderstood variable in investing: How long are you going to keep your money at work? The question isn’t that difficult.

While the future of the stock market or interest rates is always a mystery, you know quite a bit about how long you want to keep your investments at work. You can project the year your child is likely to enter college, for instance, or the approximate date when you hope to retire.

But when people invest, they often worry far more about a fund’s performance in the past year and the short-term ups and downs of the markets than they do about their goals and how long it will take them to get there.

As financial economist Jeremy Siegel told a roomful of investment planners at a recent TD Waterhouse Group Inc. conference: “You’ve got to get your clients to focus on what is the true nature of their holding period.”

The issue isn’t how long you keep your money in any given fund or stock or bond, said Siegel, a professor at the University of Pennsylvania’s Wharton School and author of the book “Stocks for the Long Run.” It’s how long you expect to stay in any of those categories of investments.

As Siegel’s research has shown, the holding period can make all the difference in the world.

A little math, using the standard deviation, a measure of risk that tracks how much the results of a given investment can vary: If you’re investing for a year, the standard deviation of stocks is much higher than that of bonds or short-term money market securities (Treasury bills).

But over 20-year or 30-year periods in the past two centuries, Siegel has calculated, stock returns actually vary less than those of bonds and Treasury bills. And their payoff has been much higher.

From 1946 to 1999, for instance, stocks averaged a real annual return that is, after subtracting the consumer price inflation rate of 7.9 percent, compared with 1 percent for long-term government bonds and 0.6 percent for short-term government bonds.

Following an ultraconservative strategy using the data since 1802, investors with a one-year holding period would want to put only 7 percent of their money in stocks. In the real world, you or I might round that off to zero, and keep all the money we expected to need 12 months from now in a money market fund.

With a five-year period, the amount you should have in stocks rises to 25 percent; with a 10-year period, 41 percent; and with a 30-year period 71 percent.

Moderate risk-takers, by Siegel’s reckoning, should put 50 percent in stocks if they want to use their money in a year. Buying for 30 years, they should put all their money in stocks.

Since World War II, stocks have returned 7.9 percent a year after inflation, compared with 4.9 percent for short-term government bonds and 1 percent for long-term government bonds.

Chet Currier is a columnist for Bloomberg News.