Working with BusinessHours methods in Salesforce Flows

BusinessHoursInvocable.apxc

public class BusinessHoursInvocable {
    
    public class Input {
        @InvocableVariable(required=true)
        public Id businessHoursId;
        
        @InvocableVariable
        public Datetime startDate;
        
        @InvocableVariable
        public Datetime endDate;
        
        @InvocableVariable
        public Long intervalMilliseconds;
        
        @InvocableVariable
        public Datetime targetDate;
    }
    
    public class Output{
        @InvocableVariable
        public Long diffMs;
        
        @InvocableVariable
        public Datetime addedDate;

        @InvocableVariable
        public Datetime addedGmtDate;

        @InvocableVariable
        public Boolean isWithinBusinessHours;

        @InvocableVariable
        public Datetime nextBusinessStartDate;
    }
    
    @InvocableMethod(label='Run Business Hours Calculations')
    public static List<Output> runCalculations(List<Input> inputList) {
        List<Output> results = new List<Output>();
        for (Input ip : inputList) {
            Output op = new Output();
            // Run BusinessHours.diff() if start and end dates are present
            if (ip.startDate != null && ip.endDate != null) {
                op.diffMs = BusinessHours.diff(ip.businessHoursId, ip.startDate, ip.endDate);
            }
            // Run BusinessHours.add() & .addGmt() if startDate and interval are present
            if (ip.startDate != null && ip.intervalMilliseconds != null) {
                op.addedDate = BusinessHours.add(ip.businessHoursId, ip.startDate, ip.intervalMilliseconds);
                op.addedGmtDate = BusinessHours.addGmt(ip.businessHoursId, ip.startDate, ip.intervalMilliseconds);
            }
            // Run BusinessHours.isWithin() & .nextStartDate() if targetDate is present
            if (ip.targetDate != null) {
                op.isWithinBusinessHours = BusinessHours.isWithin(ip.businessHoursId, ip.targetDate);
                op.nextBusinessStartDate = BusinessHours.nextStartDate(ip.businessHoursId, ip.targetDate);
            }
            results.add(op);
        }
        return results;
    }
}