@farb
Please follow the below approach(Simple .Net script):

Code(Copy and paste the below code into you flow designer workspace):
SET StartDate TO $'''01.03.24 00:00:00'''
SET Enddate TO $'''05.03.24 12:34:56'''
SET Difference TO $'''%''%'''
Scripting.RunDotNetScript Imports: $'''System''' Language: System.DotNetActionLanguageType.CSharp Script: $'''
// Parse the input strings into DateTime objects
DateTime startDate = DateTime.ParseExact(startDateString, \"dd.MM.yy HH:mm:ss\", System.Globalization.CultureInfo.InvariantCulture);
DateTime endDate = DateTime.ParseExact(endDateString, \"dd.MM.yy HH:mm:ss\", System.Globalization.CultureInfo.InvariantCulture);
// Calculate the time difference
TimeSpan timeDifference = endDate - startDate;
// Format the result in hh:mm:ss format
result = String.Format(\"{0:D2}:{1:D2}:{2:D2}\", (int)timeDifference.TotalHours, timeDifference.Minutes, timeDifference.Seconds);
''' @'name:startDateString': StartDate @'type:startDateString': $'''String''' @'direction:startDateString': $'''In''' @'name:endDateString': Enddate @'type:endDateString': $'''String''' @'direction:endDateString': $'''In''' @'name:result': $'''''' @'type:result': $'''String''' @'direction:result': $'''Out''' @result=> Difference
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