Date object and states in React.js

The most difficult part was utilizing the Date object to calculate the age and after that making each input resoponse to the multiple kinds of input, for example if the input is empty it should say "please enter a date" and the same input if it has an invalid type of date it should say, invalid day. for example February has 28 day and the 29th of February should throw and error. Another issue was the year; if the year is way in the past we would know that it's not true, and if the year is in the future we cannot calculate their age now! The hardest part after that was to design the small elements!