I should not admit how I was investigating doing this, until Marc kindly pointed out the datediff function in MySQL as outlined on this page.
To get the number of days between two dates, be it before or after.
mysql> SELECT DATEDIFF('1997-12-31 23:59:59','1997-12-30');
-> 1

or between a given date and today

mysql> SELECT DATEDIFF('1999-12-31 00:00:00',CURRENT_DATE())
-> -2442
The Date functions docs should have been investigated before now.