title: Mysql auto insert a value after substraction
body: <p>I am working on a car rental system. The table 'Hire' has fields like <code>hireDate</code>, <code>returnDate</code>, etc. I want to be able to calculate the rent and insert that value automatically into the database (same table).</p>
  
  <p>I am wondering if I can subtract these two date fields, and the value which comes out of this can be multiplied by a certain number (say 300) and automatically added to another attribute called '<code>Rent_Due</code>'. The idea is to calculate the total rent due by getting the number of days and then multiplying that by a certain number. So for every Hire that is made, I just enter the hiring date and the return date, and the Rent Due attribute is automatically filled. Is this possible by any chance?</p>
  
  <p>I dont know what more information I need to provide to seek help, kindly let me know if you might require an image of my ERD.
  Thanks!</p>