On the first round I thought it was an input, then I realized it must be a slider but I tried to use it wrong. I only persisted on my second guess and realized it was actually a slider, but it didn't work how I expected.
It was really confusing that it's called timeguessr but asks me to guess the location
Interesting, I didn't have a problem with that - maybe it's because my monitor is calibrated juuuust right so I can see the extremely-light-gray bar behind the red slider? Maybe making that more visible and making the slider not blend in with the red background of the year would be enough?
Getting the subtle elements of a UI wrong have a profound effect.