insert age of account MYSQL query - php

Question

What would be the best way to find the difference in the two dates start_date and account_add_date then insert into a column called account_age?
What I need is for a user to select a start_date say its January.
Then the date when they added account_add_date say March.
The difference is 2 months so in account_age would be 2,
then in April that 2 would be a 3. May 4, June 5 and so on
Dose anyone know how to do this ?
My current insert query MODEL
function create_bank()
{
$this->load->helper('date');
$new_bank_acc_insert_data = array(
'bank_name' => $this->input->post('bank_name'),
'interest' => ($this->input->post('interest') / 100),
'start_amount' => $this->input->post('start_amount'),
'length' => $this->input->post('length'),
'start_date' => date('Y-m-d',strtotime($this->input->post('start_date'))),
'mem_id' => $this->session->userdata('id'),
'account_add_date' => $this->current_date()
);
$insert = $this->db->insert('bank', $new_bank_acc_insert_data);
return $insert;
}
Idea on finding account age
SELECT DATEDIFF('start_date','account_add_date')

Answer 1

SELECT
(
12* (YEAR(account_add_date) - YEAR(start_date)) +
(MONTH(account_add_date) - MONTH(start_date))
) AS differenceInMonth
FROM
YOUR_TABLE
Although DATEDIFF function returns the difference in two dates in days which is more accurate comparing to the difference in month
Explanation:
Example:
start_date = 2014 March
account_add_date = 2015 January
YEAR(account_add_date) = 2015
YEAR(start_date) = 2014
MONTH(account_add_date) = 1
MONTH(start_date) = 3
So according to the query:
12 * (2015-2014) + (1-3) = 12 * 1 - 2 = 10 (Months)