CalculateNextBusinessDate.cls

global class CalculateNextBusinessDate {
    global class request
    {
        @InvocableVariable (label='Start Date' required = true)
        global DateTime givenDateTime;
        @InvocableVariable (label='Business Hours Id' required = true)
        global String businessHoursId;
    }
    global class response
    {
        @InvocableVariable (label='Next Business Date')
        global DateTime nextAvailableBusinessDate;
    }
    @InvocableMethod(label='Professor Flow | Calculate next Business Date' description='Starting from the specified target date, returns the next date when business hours are open. If the specified target date falls within business hours, this target date is returned.')
    global static List<response> getNextBusinessDate (List<request> inputParams)
    {
        List<response> responseList = new List<response>();
        DateTime nextDate;
        if(inputParams != null && inputParams.size()> 0)
        {
            for(request pf : inputParams){
                nextDate = BusinessHours.nextStartDate(pf.businessHoursId, pf.givenDateTime);
                response rs = new response();
                rs.nextAvailableBusinessDate = nextDate;
                responseList.add(rs);
            }
        }
        return responseList;
    }
}

CalculateNextBusinessDateTest.cls

global class CalculateNextBusinessDate {
    global class request
    {
        @InvocableVariable (label='Start Date' required = true)
        global DateTime givenDateTime;
        @InvocableVariable (label='Business Hours Id' required = true)
        global String businessHoursId;
    }
    global class response
    {
        @InvocableVariable (label='Next Business Date')
        global DateTime nextAvailableBusinessDate;
    }
    @InvocableMethod(label='Professor Flow | Calculate next Business Date' description='Starting from the specified target date, returns the next date when business hours are open. If the specified target date falls within business hours, this target date is returned.')
    global static List<response> getNextBusinessDate (List<request> inputParams)
    {
        List<response> responseList = new List<response>();
        DateTime nextDate;
        if(inputParams != null && inputParams.size()> 0)
        {
            for(request pf : inputParams){
                nextDate = BusinessHours.nextStartDate(pf.businessHoursId, pf.givenDateTime);
                response rs = new response();
                rs.nextAvailableBusinessDate = nextDate;
                responseList.add(rs);
            }
        }
        return responseList;
    }
}

If the Start Date is within the given business hours, the Start Date is returned instead of the next business day date.