NEW YORK -- It's one of the truisms of financial planning: trying to perfectly time the market is a fool's errand. For long-term gains, the advice goes, you should buy index funds and hold them indefinitely. Warren Buffett likes to say that his preferred holding period is "forever." But a very simplistic form of market-timing has worked for the past 11 years. It involves owning the Standard & Poor's 500 stocks, but only for the first day of every month. An S&P report recently found that someone who invested $10,000 in the S&P 500 on Dec. 31, 1999, and left the money there until Dec. 1, 2010, would have just $8,209. An investor who was in the market only on the first day of every month over the same time -- for example, buying at the close on Dec. 31 and selling at the close of the first trading day in January -- would have $13,816.

That's nearly 70 percent more than buying and holding the whole time. S&P didn't include reinvesting dividends in either scenario because of the complications of figuring out which companies paid dividends on the first trading day of the month for 11 years. But even if you include all possible dividends for the buy-and-holders, the first-day trade strategy came out 33 percentage points ahead.

The strategy appears to work because of a market quirk. Money tends to go into stocks of the first day of the month as institutional investors reopen their books for a new reporting period. It's also the day when money tends to go into 401(k) or other retirement accounts.

"It's a quirk that works," says Howard Silverblatt, a senior index analyst at Standard & Poor's. "It's hard to argue with someone who made well over 30 percent compared to someone who's down almost 20 percent."

It helped that the stock market hasn't been kind to most investors over the past 11 years. Staying in the market for only one day a month would have limited your losses during the dot-com bust or the 2008 financial crisis. The week of Oct. 6-10, 2008, for example, the S&P dropped 18 percent. First-day traders were sitting in cash and spared that agony.

Other financial experts are accepting S&P's research grudgingly. "It's not as outlandish as it first appears," says Christine Benz, the director of personal finance at fund tracker Morningstar. But she says that it might not be a strategy that most small investors can follow because of the trading costs involved and the amount of discipline that's required to stick with it month after month. She isn't planning on changing her advice any time soon.

"I don't know if I want to give up buying and holding," she says.

Silverblatt says that the first-day trade has worked over two bull markets and two bears markets, which he says gives some credence to the idea that you can time the market, but only over a long period of time.

"Timers either do the best or they go bankrupt," he says. "No one has gone bankrupt doing this."