Date of Birth From Multiple Inputs

Date of Birth From Multiple Inputs

I have a form that, at some point, asks the user "so, you are born on (day) of (month) in (year)". The question is displayed on this way for a better approach on user experience, so, (day), (month) and (year) are different inputs that user selects. The problem is on organising the information for SQL purposes (for a DATA row, that stores information like that: (YYYY-MM-DD)). 
I was hopping there is an easy way to transfer the information collected on those three input to a fourth hidden input, from which I will write using php, on my database on this order (YYYY-MM-DD).

  1.     <form action="">
  2.         <p>
  3.             so, you are born on <input type="text" id="day" placeholder="01">
  4.             of <input type="text" id="month" placeholder="january">
  5.             in  <input type="text" id="year" placeholder="2000">.
  6.         </p>
  7.         <input type="text" id="data" name="store-data" style="display:hidden">
  8.     </form>