SimpleDateFormat

http://www.yukun.info/blog/2009/02/java-jsp-gregoriancalendar-period.html
のshowTimeメソッド

String showTime(Calendar cal) {
  String str;
  str = cal.get(Calendar.YEAR) + "/"
  + (cal.get(Calendar.MONTH) + 1)  + "/"
  + cal.get(Calendar.DATE) + " "
  + cal.get(Calendar.HOUR) + ":"
  + cal.get(Calendar.MINUTE);
  return str;
}

を

String showTime(Calendar cal) {
  Date date = cal.getTime();
  SimpleDateFormat format = new SimpleDateFormat("yyyy/MM/dd HH:mm");
  return format.format(date);
}

と書けばすっきりするよ、import java.text.SimpleDateFormatを忘れずに