Calculations of two custom fields in date format


I have the two imported custom fields from my jira called “Start time” and " End time". As property:

[Issue].CurrentHierarchyMember.get('End time')

I want to create the measure called “Downtime” and it should to be difference between End time and Start time. I’ve tried functions: DateDiffHours, DateDiffDays but they return me next:

Please assist with this. Maybe I do something wrong.


This answer helped - Needed to calculate the subtraction between two dates - #2 by oskars.laganovskis -
And now I got this:
Could someone advise how can I get the result in a format like this?