For IRS purposes, in all cash acquisition when is the official date of sale of stock from investor's viewpnt?

More specifically, if the stock was a Qualified Small Business Stock and the investor is using the Section 1045 rollover provision to invest into another QSBS to defer his/her gains, does the 60 day countdown start when the acquisition transaction closes or when the investor receives his/her check. Please provide references, thanks.|||when it closes