FIGURE 2: Call start and end times
username	firstminute	lastminute	diff
user1	2003-02-12 21:30:00	2003-02-12 22:29:00	37662
user2	2003-02-12 23:00:00	2003-02-13 22:59:00	37662
user3	2003-02-04 23:00:00	2003-02-07 22:59:00	37501