Working duration is calculated against different working hours configurations so displaying the working duration in days has no valuse.
One working configuration can be 8 hours, another 12 hours and another 24 hours so showing the working duration in days has different meaning for each of those values effectively 1d could mean 8, 12 or 24 hours and there is no way to tell them apart when presented in days.
It's simpler and better to simply always show working duration in hours and minutes on issues as well as report on that since customers want to be able to see how many issues got resolved with x time frame. ( customers love to claim and complain that all issues are solved just before the sla times runs out so at least we need to be able to report 20% issues got resolved within 2 hours , 40% within 4 hours and 40% within 8 hours and your sla contract is 16 hours etc )
btw is there any sql query I can perform to fetch working duration for an issue in the database?