This has not been made clear, but I take the position that for 1031 exchanges, the 180-day period begins upon receipt of boot. It's more clear when the exchange crosses tax years due to the (j) regs under 1031, but it's less clear when the exchange occurs all in the same taxable year. But either way, my position has been that receipt of boot starts the QOZ timer, rather than sale of relinquished.