I’ve been doing this for a few years already and I can’t believe I only thought of sharing it now.
The idea came when I was trying to go through medicines and skin care products to weed out expired items. Then I found out that some products, instead of having an expiry date on them, had a period-after-opening-symbol printed on the label.
I had no way of knowing if the after-opening period had already lapsed — for sure that would come before expiration date.
Since then, I always keep a permanent marker ready so I can write the month and year when I first opened or used a product.
Tip: I write the date at the bottom of the bottle so that it is less likely to fade over time.