Once a packet of food has been opened it’s really hard to tell how long it will last or whether it’s actually on it’s last legs and you shouldn’t be consuming it at all.
For example, I got sick of opening a packet of cheese and going to use it again wondering how long it was in the fridge (if it wasn’t noticeably moldy already). I still use a clip to seal the bag but I then put that bag into a zip-lock bag and write the date I opened it. Usually food labels tell you how long you should keep it in the fridge once opened and this is an easy way to remember.
I also write the date I opened a carton of stock so that I can use it up within the 3 or so days it will last in the fridge. With our busy lifestyle we don’t have time to remember what was opened when. Take the stress out of it and start keeping track.