Age calculator(html, css, js)

This is my first upload of a website I've programmed using JavaScript! This one was probably the biggest challenge for me so far, figuring out how I could print the error messages under the input boxes, printing out the error messages and actually getting the results to display. I'm familiar with the basic fundamentals of JavaScript, but it's also had the biggest learning curve out of the languages I've worked with, other than C++. One thing I'm still having trouble with, is actually getting the day to display correctly, because it doesn't actually seem to match the actual number of days. So I think my question is, how would you fix things like that? So anyway, feel free to check this out and offer me any suggestions or feedback!