Salesforce Apex Datetime Utils

DatetimeUtils.cls

public without sharing class DatetimeUtils {

    public static String getDayOfMonthSuffix(Integer n) {

        if (n == null) {
            return '';
        }

        if (n >= 11 && n <= 13) {
            return 'th';
        }

        Integer modResult = Math.mod(n, 10);        
        if (modResult == 1) { 
            return 'st'; 
        } else if (modResult == 2) { 
            return 'nd'; 
        } else if (modResult == 3) { 
            return 'rd'; 
        } else { 
            return 'th';
        }
    }
}