Taking the acronyms in order. A qualified opportunity fund, QOF, is the investment vehicle that holds qualifying assets in a designated opportunity zone. A qualified opportunity zone business, QOZB, is an operating business the fund can own instead of owning property directly, and most real estate deals are actually structured with the fund owning a QOZB that owns the project. You care about that layer because the QOZB has its own tests to pass, and if it fails them the fund's compliance is the thing that breaks, which affects you as an investor even though you never touch it.
The 180 day window is the period you have to move an eligible gain into a QOF. The start date depends on the kind of gain. A straight sale of stock is one case, a gain reported to you on a K-1 from a partnership is another, and the two don't start counting on the same day. That is genuinely a question for your own tax preparer with your actual documents in front of them, because getting the start date wrong is not fixable after the fact.
The part that trips up people reading their first set of fund documents: you're investing a gain, and only the gain gets the deferral treatment. If you sell for 500,000 and 200,000 of that is gain, the 200,000 is what carries the benefit. If you put in 300,000, the extra 100,000 sits as a separate non-qualifying piece with ordinary tax treatment on its own appreciation. Some funds accept mixed money and some don't, so check the subscription agreement.
