Return row in given date
You can do it this way

-- Tasks that were running at specific (e.g. 2014-02-01 12:15:00) point in time 
SELECT id, name, task_start, task_finish
  FROM tasks
 WHERE '2014-02-01 12:15:00' BETWEEN task_start AND task_finish;

-- Tasks that are running right now
SELECT id, name, task_start, task_finish
  FROM tasks
 WHERE NOW() BETWEEN task_start AND task_finish;


Here is SQLFiddle demo