Custom Date tag

Custom Date tag: custom date based on pattern format. Default date is current day.
　　<CUSTOMDATE[+,-][value][scale],Pattern>
　　User can define the date format by customize the parameter Pattern.
　　Scale is only recognized within below option:
　　　　“Y” for year, “M” for month, “D” for day
Sample:
　　Suppose today is 16 Dec 2014
　　　　<CUSTOMDATE+1M,MMM yyyy> return as “Jan 2015”
　　　　<CUSTOMDATE-1D,ddMMyyyy> return as “15122015”
　　　　<CUSTOMDATE-2Y,dd*MMMMyyyy> return as “16*December2012”

　　　/**
     * @param input
     *            <CUSTOMDATE[+,-][number][scale],Pattern>
     * @return custom date based on pattern format. default date is current day
     */
    private String customDate(String input) {
        input = input.substring(1, input.length() - 1);
        String pattern = input.split(",")[1].trim();
        input = input.split(",")[0].trim();
        Date now = new Date();
        SimpleDateFormat ft = new SimpleDateFormat(pattern);
        GregorianCalendar gc = new GregorianCalendar();
        gc.setTime(now);
        int len = "CUSTOMDATE".length();
        if (input.length() == len) {
            return ft.format(now);
        } else if (input.length() < len) {
            return "The format you input is incorrect! Please make sure the format is <CUSTOMDATE[+,-][number][scale],Pattern>";
        } else {
            String math = input.substring(len, len + 1);
            int value = Integer.parseInt(input.substring(len + 1,
                    input.length() - 1));
            String scale = input.substring(input.length() - 1);
            if (math.equals("-")) {
                value = -value;
            } else if (!math.equals("+")) {
                return "Math is wrong. Can only use +/-.";
            }
            int field = 0;
            if (scale.equalsIgnoreCase("Y")) {
                field = 1;
            } else if (scale.equalsIgnoreCase("M")) {
                field = 2;
            } else if (scale.equalsIgnoreCase("D")) {
                field = 5;
            }
            if (field == 0) {
                return "The scale is wrong. Can only use Y/M/D.";
            }
            gc.add(field, value);
        }
        return ft.format(gc.getTime());
    }