CalculateBusinessHoursDiff.cls

global class CalculateBusinessHoursDiff {
    global class request
    {
        @InvocableVariable(label='Business Hours Id' required = true)
        global String businessHoursId;
        @InvocableVariable(label='Start Date' required = true)
        global DateTime startDate;
        @InvocableVariable(label='End Date' required = true)
        global DateTime endDate;
    }

    global class response
    {
        @InvocableVariable (label='Result (in millisecond)')
        global long diffResult;
        @InvocableVariable (label='Result (in second)')
        global long diffResultSec;
        @InvocableVariable (label='Result (in minute)')
        global long diffResultMin;

    }
    @InvocableMethod(label='Professor Flow | Calculate Business Hours Difference' description='Returns the difference in milliseconds,second or minute between a start and end Datetime based on a specific set of business hours.')
    global static List<response> getMilliSecondDiff(List<request> inputParams)
    {
        List<response> finalList = new List<response>();
        long diffinms = 0;
        //use utility method in businesshours
        if(inputParams != null && inputParams.size() > 0)
        {
            for(request pf: inputParams)
            {
                diffinms = BusinessHours.diff(pf.businessHoursId,pf.startDate,pf.endDate);
                response resp = new response();
                resp.diffResult = diffinms;
                resp.diffResultSec = diffinms / 1000 ;
                resp.diffResultMin = diffinms / 60000 ;
                finalList.add(resp);
            }
        }
        return finalList;
    }
}

CalculateBusinessHoursDiffTest.cls

@isTest
global class CalculateBusinessHoursDiffTest 
{
    @isTest static void check_diffInMilliSeconds() 
    {
        Profile sysadminprofile = [select Id, name from profile where Name='System administrator' limit 1];
        User adminUser = [select Id,Name from user where profileId= :sysadminprofile.Id and isactive= True limit 1];
        BusinessHours bhoursId = [select Id from BusinessHours where Name='Default' limit 1];
        System.runAs(adminUser)
        {
            List<CalculateBusinessHoursDiff.request> inputparams = new List<CalculateBusinessHoursDiff.request>();
            CalculateBusinessHoursDiff.request pf = new CalculateBusinessHoursDiff.request();
            pf.startDate = Datetime.now();
            pf.endDate = Datetime.now().addDays(1);
            pf.businessHoursId = bhoursId.Id;
            inputparams.add(pf);
            Test.startTest();
            List<CalculateBusinessHoursDiff.response> response = CalculateBusinessHoursDiff.getMilliSecondDiff(inputparams);
            Integer total_duration_in_seconds = (response[0].diffResult/1000).intValue();
            Test.stopTest();
            System.assertEquals(86400,total_duration_in_seconds );
            
        }
    }
}