Does the effectiveness of cosmetics decrease if I use them for a long time after opening?

A while ago, while organizing my vanity, I found a serum that I had bought a long time ago and forgotten about for a while.

There is still quite a lot left, and the smell and color don't seem to have changed much, so I wondered if it was okay to just keep using it.

When you buy cosmetics, you apply them diligently at first, but if you become interested in another product, you end up putting the one you were using aside for a while, don't you?

I also tend to buy various products that are said to be good, but I found that actually using them all the way to the end is not as easy as I thought.

But then, it suddenly occurred to me that cosmetics also have expiration dates, and that the usage period changes after opening.

I'm confused whether it's okay until the date written on the packaging, or if I need to calculate the usage period separately from the moment I open the lid.

Especially for products where ingredients are important, such as Vitamin C serums or functional creams, can their effectiveness decrease over time?

Even if it looks fine on the outside, I hesitate to use it blindly because I think the ingredients might have already changed.

But it feels like a waste to throw it away just because there is a little left.

Does anyone happen to write down the opening date or manage the usage period for each product?

I would love to hear from those of you who, like me, switch between multiple cosmetic products. Please share your experiences on how you manage them and just how important it is to adhere to the expiration date after opening. 🙂