Summary:

The customer had set up a business calendar and default SLA, but were finding that the Duration value in the Tracking tab was showing unexpected values.

Symptoms:

In this case, the customer had entered a workday running from 12:00AM (midnight) to 12:00PM (midday), when a 24 hour workday was required. The number of days elapsed on the Tracking tab was the duration divided by 720 minutes (12 hours), while the minutes were Duration mod 720.

Work around / Resolution:

A 24 hour workday can't really be entered by default, but we can come pretty close. The trick is to enter a new value for the Minutes dropdown so that you can run a work day from 00:00 to 23:58.
The value is entered in Administration -> Customisation -> Translations; details are below:

Caption family: Minute
Caption code: 59
Caption family type: Choices
US/UK etc translation: 59
Caption Order: 13

The durations for Cases are updated immediately once the calendar and default SLA have been amended.