Since the gain comes from a pass-through entity, you have 180 days from Dec. 31, 2019 to invest the gain in an Opportunity Zone fund and get the tax benefits of deferral and future tax-free capital gains. Because your basis is increased by 10% after five years of holding and another 5% after seven years, you would need to invest the gain prior to Dec. 31, 2019 to get the full 15% step-up in basis prior to 2016. However, don’t rush into an investment for just a 5% step-up. The big pay off will be getting future capital gains tax free after 10 years.