SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
1.Calendar 转化 String 
//获取当前时间的具体情况,如年,月,日,week,date,分,秒等 
Calendar calendat = Calendar.getInstance();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
String dateStr = sdf.format(calendar.getTime());
  
2.String 转化Calendar
String str="2010-5-27";
SimpleDateFormat sdf= new SimpleDateFormat("yyyy-MM-dd");
Date date =sdf.parse(str);
Calendar calendar = Calendar.getInstance();
calendar.setTime(date);
  
3.Date 转化String
SimpleDateFormat sdf= new SimpleDateFormat("yyyy-MM-dd");
String dateStr=sdf.format(new Date());
  
4.String 转化Date
String str="2010-5-27";
SimpleDateFormat sdf= new SimpleDateFormat("yyyy-MM-dd");
Date birthday = sdf.parse(str);
  
5.Date 转化Calendar
Calendar calendar = Calendar.getInstance();
calendar.setTime(new java.util.Date());
  
6.Calendar转化Dateate 转化Calendar
Calendar calendar = Calendar.getInstance();
java.util.Date date =calendar.getTime();
  
7.Date 转成 String
System.out.println(sdf.format(new Date())); 
  
8.String 转成 Timestamp
Timestamp ts = Timestamp.valueOf("2011-1-14 08:11:00");
  
9.Timestamp 转成 String
sdf.format(ts);
  
Timestamp和Date多数用法是一样的。
10.Date 转 TimeStamp
SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String time = df.format(new Date());
Timestamp ts = Timestamp.valueOf(time);
  
11.日期比较大小
String ti="2010-11-25 20:11:00";
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); 
Date time=sdf.parse(ti);
String ti2="2011-11-26 20:11:00";
Date time2=sdf.parse(ti2);
int c=ti2.compareTo(ti);
if(c>0){
    System.out.println(ti+"大");
}else if(c==0){
    System.out.println("一样大");
}else{
    System.out.println(ti2+"大");
}
  
12.Date/ Timestamp 转成 Calendar 
Calendar calendar = Calendar.getInstance();
calendar.setTime(startDate);
calendar.add(Calendar.YEAR, 2);   //日期加2年
System.out.println(calendar.getTime());
calendar.add(Calendar.DATE, -30);     //日期加30天
System.out.println(calendar.getTime());
calendar.add(Calendar.MONTH, 3);  //日期加3个月
System.out.println(calendar.getTime());