获得当前时间到上个月的当前时间

Timestamp transDate = new Timestamp(System.currentTimeMillis()); // 获取系统当前时间??
?
// 构建日历对象,获取上月的当日??
Calendar cal = Calendar.getInstance();??
cal.setTime(transDate);??
cal.add(Calendar.MONTH, -1);??
// 将日历对象转变为Timestamp??
transDate = new Timestamp(cal.getTimeInMillis());