What is the 180-day eligibility window to make a QOZ investment?

In general, the 180-day period begins on the day the gain is recognized by the taxpayer.

Special considerations for how the 180-day rule applies if the taxpayer receives their subject gains from an investment in a partnership or an investment in another type of pass-through entity:

A Schedule K-1 is issued by a flow-through or pass-through entity, which could be a partnership, S corporation or trust—any of which could have eligible gains that would be reported on a Schedule K-1 to their partners, shareholders or beneficiaries.
A taxpayer receiving the Schedule K-1 can choose to begin the 180 days to invest their eligible passed-through gains into a QOZ fund on any one of three following dates:
The last day of the flow-through entity’s taxable year
The same date the flow-through entity realized the actual eligible gain at the entity level, i.e., the same date that the flow-through entity’s180-day period begins
The due date for the flow-through entity’s tax return without extensions for the taxable year in which the entity realized the gain

Special considerations for how the 180-day rule applies if the taxpayer will be receiving installment or earnout payments from a business sale:

Each installment payment or earnout payment the taxpayer receives will have a separate 180-day period. If the business sale occurred on March 1, 2023, for example, and the first payment was also received in 2023, the taxpayer’s 180-day period for that period would begin on the last day of the business’s fiscal year tax year in the year the sale occurred.
Each subsequent installment/earnout payment would have a separate 180-day period that would begin the day that each subject installment payment is received.

A taxpayer must first realize the capital gain before reinvesting some or all of that gain into a QOZ fund. The IRS considers a taxpayer to have “made the QOZ investment” on the date that the taxpayer wires the money for their investment commitment to the QOZ fund.