setting date field to not auto update

I have column A as a text field containing names, column B has numerical data and column C has dates in it.

What i am trying to do is set up the spreadsheet so that if i update the numerical entry in column B the date in C will update to the current date automatically.

At this time i can't figure out how to fix the date so that it doesn't update to todays date when i simply open the spreadsheet, do i have to put in the date manually in order to 'fix' a start date or is there another way?