Usually, you do this kind of thing in a query, but MySQL isn't very good with intervals (it would be very easy with PostgreSQL). You could convert it to unix timestamp, then it would give the number of seconds between the two dates :
SELECT UNIX_TIMESTAMP() - UNIX_TIMESTAMP(your_datetime_column);
I thought about DATEDIFF, but it only returns the number of days between the two dates.
You can do it in PHP, for instance, with DateTime class :
$date1 = new DateTime();
$date2 = new Datetime('2010-11-26 12:00:00');
var_dump($date1->diff($date2));
(There's a procedural way to do this, if you're not a fan of OOP.)
This is definitely the solution I'd use if I can't do it with the RDBMS. DateTime::diff returns a DateInterval object, which contains the number of seconds, minutes, hours, days, etc. between the two dates.
You could also do it with timestamps in PHP :
$num_sec = time() - strtotime('2010-11-26 12:00:00');
Which would return the same thing as the SQL query.