Excel Query 2

I have a spreadsheet where people input a date and time to show when work is completed I originally devised a check box on the spreadhseet so when they clicked the checkbox a cell would display the time and date but there is over 2,000 rows to the sheet and inputting 2,000 tick boxes would be a major ballache. Is there a formula I could use that ..say if somebody input OK in cell J2, cell J3 would display the time and date as in a =NOW() way but making sure cell J3 stayed at the original time 'OK' was originally input and not get updated everytime the spreadsheet was saved or re-opened.