日期处理笔记

获取上个月

java.text.SimpleDateFormat df = new java.text.SimpleDateFormat("yyyy-MM");
java.util.GregorianCalendar gc = (java.util.GregorianCalendar) Calendar.getInstance();  
gc.setTime(new java.util.Date());  
gc.set(Calendar.DAY_OF_MONTH, 1);  
String day_first = df.format(gc.getTimeInMillis()-24*60*60*1000);  
System.out.println(day_first);