A lot of confusion has been created by the IRS's rules for "Section 1231 gains," that is, gains from the sale or property used in a trade or business. The start date for those gains is different from the start date for most "capital gains". In brief: If you have gains from the sale of stock or other assets not used in a trade or business (I’ll call these "portfolio gains") then the start date is the date of sale. If you have gains from the sale or trade or business assets (e.g., a residential building that you own and rent out, or your company's corporate headquarters; I'll call these "Section 1231 gains") then the start date is Dec. 31 of the year of the sale. In the case of Section 1231 gains, you have to wait till Dec. 31 to see how much of these kinds of gains (and losses) you have, and you can only get favorable OZ treatment for the net gain, if any. For example, if you have a $5 million gain from the sale of a building you rent out in March 2019 and a $3 million loss from the sale of another rental property in October, 2019, then on Dec. 31, 2019, you will compute that you have a net Section 1231 gain of $2 million, which is the amount that is eligible for the favorable OZ treatment. Note that Dec. 31, 2019, is the last possible date to qualify for the full 15% reduction in gain under the code provision, and it is also the first possible date to qualify for OZ investing in the case of Section 1231 gains. So, that singe day may be a big deal for people who have Section 1231 gains. The IRS has grandfathered people who made their investments of Section 1231 gains immediately following the investment in 2018, provided they didn't exceed the net amount of the gain as finally computed on Dec. 31. The IRS didn't say what happens if the investment did exceed the net Section 1231 gain. Presumably, the excess is just like any other investment in excess of gain. A lot of people have complained to the IRS about the Section 1231 rule, both because of the delay until Dec. 31 and also because of the netting rule (as opposed to using the gross amount of the gain, and ignoring any Section 1231 losses). So, there is always a chance that the IRS will change its rule on account of one or both of these two issues.