title: Operation with dates in Grocery CRUD and Codeigniter
body: <p>In table Employees I have columns: start_date(dd/mm/yyyy), end_date(dd/mm/yyyy) and period(dd/mm/yyyy). 
  I want to put calculation in column period = end_date - start_date.
  I don't now haw to take data from columns start_date and end_date and write operation for column period. Please help me to solve my problem.</p>