What is your system for dealing with old food in the fridge?
Do a fridge sweep on a weekly basis.
If I happen to notice a science experiment, I'll throw it out.
Push it to the back and hope that it doesn't develop into an intelligent life form.
Some other fabulous system that I will explain in comments.
Do you have some system for keeping track of when food items were first opened?
Yes, and I will happily comment and tell you what it is.
No, I am not nearly that organized.
I never even thought of doing something like that.
How about leftovers?
Anything not eaten within a few days gets thrown out.
Keep them for at least a week or until they start smelling funny.
Mmm, science experiments.
I don't believe in leftovers.
Other, see comments.