Sharpies are our friend. I always keep a sharpie in my kitchen to write the date I opened something. In this shot, I opened my organic chicken stock today. This way I know how long a container has been open. This goes well for leftovers too. If you’re using reusable containers and don’t want permanent marker, try a dry erase marker. If you write in an area that you won’t be grabbing, it should stay well enough.
I also use sharpies in other places around my home too. I put the open date on my laundry detergent so I can see how fast we are going through it and how many I should pick up to survive until the next shopping trip. So, grab a sharpie and keep it in your kitchen too!