ktp_util_CaseExpirationDateCalculator calExpirationDate = new ktp_util_CaseExpirationDateCalculator(2,40,2018-01-01);
                    Date newExpirationDate = calExpirationDate.getNewExpirationDate();
                    system.debug('The newExpirationDate is ' + newExpirationDate);

public with sharing class ktp_util_CaseExpirationDateCalculator {
    
    private Integer quantity { get; set; }
    private Integer duration { get; set; }
    private Integer totalDuration { get; set; }
    private Date currentExpirationDate { get; set; }
    private Date newExpirationDate { get; set; }  

    //Class constructor
    public ktp_util_CaseExpirationDateCalculator(Integer quantity, Integer duration, date currentExpirationDate){ 
    
        this.quantity = quantity;
        this.duration = duration;
        this.totalDuration = calculateLengthOfExtension();
        this.currentExpirationDate = currentExpirationDate;
        this.newExpirationDate = calculateNewExpirationDate ();

    }
    
    //Calculate length of extension by multiplying quantity by duration
    private Integer calculateLengthOfExtension() {
        
        Integer validQuantity = validateQuantity();
        Integer lengthOfExtension;
        
        lengthOfExtension = validQuantity * duration;
        
        return lengthOfExtension;
        
    }
    
    //Validate that quantity is greater than 0; otherwise return 1
    private Integer validateQuantity() {
        
        if(quantity == null) {
            return quantity = 1;
        } else {
        	return quantity;
       	}
    }
    
    //Calculate new expiration date by adding totalDuration to currentExpirationDate
    //expirationDateCheck checks to see if date is in the past and uses today if true; otherwise the enroll exp. date
    private Date calculateNewExpirationDate() {
        
        Date expirationDateCheck;
        expirationDateCheck = (currentExpirationDate < system.today()) ? system.today() : currentExpirationDate;
        Date calculatedNewExpirationDate;
        calculatedNewExpirationDate = expirationDateCheck + totalDuration;
       
        return calculatedNewExpirationDate;
    }
    
    //Get new expiration date
    public Date getNewExpirationDate() {
        
        return newExpirationDate;
        
    }
}