MYSQL measuring time

Hi there,
I would like to measure the time spent doing a task during a specified period (e.g. 8 hr shift.) The table is structured as follows:

id, emp_no, task_id, start_date;

Each time someone starts a new task they submit to the above table. The start_date is also the end_date for the previous task.

Is this table design adequate? How do I query the db so as to get the total time per task and the time each person spent doing a task?

Thanks in advance!!

You need separate fields for start date and end date. What if the person finishes a task, goes to lunch, and then starts a new task? You've just counted lunch as part of the task time of the previous task.