I don’t wear makeup every day, in fact I hardly ever wear makeup, so if I were to go by the use by date I’d be throwing out hundreds of pounds worth of barely used makeup every 6 months. I do replace eyeliners and mascaras regularly, but eyeshadow palettes are stored in a place with minimal humidity, they’re sterilised every month, and used until I think they’re no good anymore.
I take absolutely no notice of use by dates. If something smells ok, looks ok, hasn't seperated or gone funny and applies the same way it always has then I use it.
I'm only putting products on my own face though and am not in any way sensitive. If I had issues with sensitivity, ever had any kind of infection or was using my make up on others it would be a different story.
If it doesn't smell, hasn't separated and still looks ok, then I'll use it. I'm more careful about what I put on my face, as it can be sensitive sometimes, but in general I just ignore the use by dates...but not for too long though, a few months is fine, but a year or more then I'd be a bit dubious.
I recently found some old lipsticks, and I was like great, I love those colours, but when I tried them on, they smelt horribly waxy, and off...so I chucked them.
I check the use by dates for all my skincare and liquid make up. The eyeshadows and powder make up have longer lifespan than a yeah so I tend to leave the eyeshadows untill i think they starting to change colour or the consistency.