rocknbil - 1:33 am on Feb 15, 2010 (gmt 0)

select username from chat_users where login_time >= date_sub(now(), interval 5 minute) order by username asc, login_time desc;

Like that?

In mysql the only difference between timestamp and datetime is the range of dates you can enter into a timestamp field, and of course, timestamp is automatically added. The formats are identical.