public class T {
public static void main(String[] args) throws Exception {
Date d = new Date();

String abc ="2011-11-20 15:15:20";
SimpleDateFormat dt = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Date x =dt.parse(abc);
String y = dt.format(x);
Date xx =Calendar.getInstance().getTime();
GregorianCalendar gc= new GregorianCalendar();
gc.setTime(x);
gc.add(5, -2);
System.out.println(gc.getTime());
}
}

package com.unicom.common;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.GregorianCalendar;

public class CompareDate {
public String getNow2String(String format) {
  Date now = new Date();
  SimpleDateFormat fmt = new SimpleDateFormat(format);
  return fmt.format(now);
}

public Date getNow() {
Date now = new Date();
return now;
}

public Date getNow2Date(String format) throws ParseException {
  String dt = getNow2String(format);
  DateFormat fmt = new SimpleDateFormat(format);
  return fmt.parse(dt);
}

public Date string2Date(String dateStr, String format)
   throws ParseException {
  DateFormat fmt = new SimpleDateFormat(format);
  return fmt.parse(dateStr);
}

public String date2String(Date orgindate, String format)
   throws ParseException {
  DateFormat fmt = new SimpleDateFormat(format);
  return fmt.format(orgindate);
}

public Date dateChangeByDay(String orginDate, String format, int amount)
   throws ParseException {
  GregorianCalendar gc = new GregorianCalendar();
  Date dt = string2Date(orginDate, format);
  gc.setTime(dt);
  gc.add(5, amount);
  return gc.getTime();

}

public Date dateChangeByMonth(String orginDate, String format, int amount)
   throws ParseException {
  GregorianCalendar gc = new GregorianCalendar();
  Date dt = string2Date(orginDate, format);
  gc.setTime(dt);
  gc.add(2, amount);
  return gc.getTime();

}

public Date dateChangeByYear(String orginDate, String format, int amount)
   throws ParseException {
  GregorianCalendar gc = new GregorianCalendar();
  Date dt = string2Date(orginDate, format);
  gc.setTime(dt);
  gc.add(1, amount);
  return gc.getTime();

}

public boolean greaterThan(Date date1, Date date2) {
return date1.before(date2);
}

public boolean lessThan(Date date1, Date date2) {
return date1.after(date2);
}

public boolean greaterThan(String date1, String date2, String format)
   throws ParseException {
  Date dateA = string2Date(date1, format);
  Date dateB = string2Date(date2, format);
  return dateA.before(dateB);
}

public boolean lessThan(String date1, String date2, String format)
   throws ParseException {
  Date dateA = string2Date(date1, format);
  Date dateB = string2Date(date2, format);
  return dateA.after(dateB);
}
}

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