The clock is ticking for wealthy investors to grab one particular morsel of a colossal tax incentive. Meanwhile, the incentive, Opportunity Zone funds, may face a time bomb in the Biden administration’s proposed tax increases. The funds, which invest in real estate in low-income or economically distressed communities, sport three lucrative benefits — two that shrink and defer old tax bills on an investor’s prior gains that are rolled into the funds, and a monster one that eliminates future tax on the fund’s eventual profits.