I'm looking for a way to automatically add the date and time to a cell when I add data to another cell. so

I'm creating a log spreadsheet. During our test we log all the incidents, problems, or just plain notes that occur. The spreadsheet consist of three columns, Col b = date and time, col C=type entry, col D=description. When I enter a type in column c, I want to populate column b in the same row with the date and time. I tried using the NOW function, but when I type in a value in column C on the next row, it changes all the dates/times in both rows. Is there a way to hold the date/time once it has been populated.