Dates constantly changing

This is probably a simple solution, but one I just can't figure out. I need a formula that will make a certain cell, say F8, display today's date, but once today is over, I want it to stay the same date. The problem I'm having with this is, the date will change in the cell as my system clock rolls past midnight. Once that date is in the cell, I want it to stay there an not change. Any ideas?