The article explores how to build a smart investment portfolio over time, considering transaction costs. By using a special math problem, the researchers found that the best strategy involves adjusting the mix of stocks and bonds to stay within certain limits. They discovered that there is a critical time period where you may not reach your expected return if it's too short. Also, it's best not to buy stocks too close to their maturity date. These findings show that the length of time you plan to invest is crucial for making the most of your money.