How is that date determined?

There is no particular formula, just what both companies agree on that would be convenient. The date however, is usually a date in the future ahead of the announcement. For eg if a buyout was announced today for say $6/sh a N/R would mention that shareholders of record on Aug 10/12 ( arbitrary date ) would receive $6 in cash (or whatever eq.) on that date. Your brokerage will automatically do the conversion for you on that date. So investors can either hold onto their stock for that date or sell before then as the stock price would immediately jump up to reflect this value upon the NR announcing the buyout. And of course you can buy stock right up to the date of the announcement at lower prices. Hope this helps.