Responsive age calculator app built with React (no figma file)

Unsure about the days calculation

I spent a lot of time working on the calculations to compute the age in years, months, days based on a birth date. Initially, I incorrectly calculated the days. At first I thought the days represented the number of days since one's birthday.

However, after checking the design specs it looks more like it's number of days since the day of your birth.

For example, at the time of this writing it is June 11. If you were born on the the 4th of June, then it's been 7 days since your birthday so you would be XX years, 0 months, and 7 days.

Feedback welcome

Hopefully, I've (re)interpreted this correctly and properly implemented the code for it. If not, I would greatly appreciate feedback on my error and how to improve it.

Other feedback related to styling or best practices for HTML/CSS/JS/React is also welcome.

Thank you.