flexbox, DOM

*Sorry for my bad english So, I compared my project with others AgeCalculators and there is a small difference in days. I know that my method is far from being the best, I searched for some tutorials to see the usual approach to this challenge, but I would like to try my own. The thing is, I don't know why there is a difference in 'days', maybe I'm missing something or I shouldn't use parseInt or I should use another thing. What am I missing, logic stuff, calc stuff? For mobile, I used position relative and absolute to put the button in the middle, but my values were insane, looks like I did something wrong, idk. Last thing, isn't :focus on input should be working? I put it but isn't working, my border-color keep the same.