Age calculator using JavaScript

What are you most proud of, and what would you do differently next time?

I think the most proud thing is that I control multiple conditions and give the error message for the wrong input value.

What challenges did you encounter, and how did you overcome them?

Challenges I faced. 1/ I faced challenges when I checked multiple conditions at the same time. 2/ Milliseconds to year, month, and day conversion. 3/ Handling the error state and showing the error result. 4/ Checking the month was 30 days or 31, and also leap year checking

How did I overcome this. 1/ Search in Google and know the time difference given in milliseconds. 2/ For the error result showing I just add a class name with JS and add a style with JS.

What specific areas of your project would you like help with?

I have tried to make this perfect. If anyone finds any bug then please comment on this post.