怎样用java获得想要的日期？

比如说我有一个起始日期，我想要在这个日期的基础上加上或减去相应的年，月，日，然后得到一个目标日期。
例如起始日期是2015-1-1，我想得到这个日期加上九个月以后的日期
请问如果用Calendar来草错的话该怎么做？

 public static void main(String[] args) throws Exception {

        SimpleDateFormat sdf=new SimpleDateFormat("yyyyMMdd");
        String str="20150101";
        Date dt=sdf.parse(str);
        Calendar rightNow = Calendar.getInstance();
        rightNow.setTime(dt);
        //rightNow.add(Calendar.YEAR,-1);//日期减1年
        rightNow.add(Calendar.MONTH,9);//日期加9个月
        //rightNow.add(Calendar.DAY_OF_YEAR,10);//日期加10天
        Date dt1=rightNow.getTime();
        String reStr = sdf.format(dt1);
        System.out.println(reStr);

    }

}

Calendar c = Calendar.getInstance();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
System.out.println(sdf.format(c.getTime()));
c.add(Calendar.DATE, -7);
System.out.println(sdf.format(c.getTime()));

如果用oracle数据库的话，可以直接用sql来实现

select add_months(to_date('2015-01-01','YYYY-MM-DD'),9) from dual;

注意一下，如果你是2015-01-31，加一个月的话，可能结果不是你想要的

使用org.joda.time.DateTime这个类吧它有plus方法，可以增加你需要的，不用你再计算是不是月份的最后一天啊，等等那样了，你直接调用方法就可以实现效果。
DateTime startdt = new DateTime(begin);
DateTime enddt = new DateTime(end);
/*下面这些是固定的上班时间*/
//早上上班时间 8:30

Date beginworkdate = (Date) begin.clone();
beginworkdate = DateUtils.setHours(beginworkdate, 8);
beginworkdate = DateUtils.setMinutes(beginworkdate, 30);
DateTime workbegin = new DateTime(beginworkdate);

danielinbiti给出的答案很好，可以满足需求了。如果涉及到跨时区的问题，需要考虑使用TimeZone.

Java 获取指定日期的方法总结
转：http://bdcwl.blog.163.com/blog/static/765222652009104171521/

格式化日期 String-->Date 或者 Data-->String

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");

Date date = sdf.parse("2009-11-04");//String-->Date

String sdate = sdf.format(date );// Data-->String ===============================================================

package com.hefeng.test;
import java.text.DateFormat;
import java.text.ParsePosition;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;
public class TimeTest {
//用来全局控制上一周，本周，下一周的周数变化
private int weeks = 0;
private int MaxDate;//一月最大天数
private int MaxYear;//一年最大天数
/**

@param args */ public static void main(String[] args) { TimeTest tt = new TimeTest();

System.out.println("得到6个月后的日期:"+tt .getAfterMonth(6);
System.out.println("获取当天日期:"+tt.getNowTime("yyyy-MM-dd"));
System.out.println("获取本周一日期:"+tt.getMondayOFWeek());
System.out.println("获取本周日的日期~:"+tt.getCurrentWeekday());
System.out.println("获取上周一日期:"+tt.getPreviousWeekday());
System.out.println("获取上周日日期:"+tt.getPreviousWeekSunday());
System.out.println("获取下周一日期:"+tt.getNextMonday());
System.out.println("获取下周日日期:"+tt.getNextSunday());
System.out.println("获得相应周的周六:"+tt.getNowTime("yyyy-MM-dd"));
System.out.println("获取本月第一天日期:"+tt.getFirstDayOfMonth());
System.out.println("获取本月最后一天日期:"+tt.getDefaultDay());
System.out.println("获取上月第一天日期:"+tt.getPreviousMonthFirst());
System.out.println("获取上月最后一天的日期:"+tt.getPreviousMonthEnd());
System.out.println("获取下月第一天日期:"+tt.getNextMonthFirst());
System.out.println("获取下月最后一天日期:"+tt.getNextMonthEnd());
System.out.println("获取本年的第一天日期:"+tt.getCurrentYearFirst());
System.out.println("获取本年最后一天日期:"+tt.getCurrentYearEnd());
System.out.println("获取去年的第一天日期:"+tt.getPreviousYearFirst());
System.out.println("获取去年的最后一天日期:"+tt.getPreviousYearEnd());
System.out.println("获取明年第一天日期:"+tt.getNextYearFirst());
System.out.println("获取明年最后一天日期:"+tt.getNextYearEnd());
System.out.println("获取本季度第一天到最后一天:"+tt.getThisSeasonTime(11));
System.out.println("获取两个日期之间间隔天数2008-12-1~2008-
9.29:"+TimeTest.getTwoDay("2008-12-1","2008-9-29"));
}

/**

得到指定月后（前）的日期参数传负数即可

public static String getAfterMonth(int month) {
Calendar c = Calendar.getInstance();//获得一个日历的实例

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");

Date date = null;

try{

date = sdf.parse("2009-11-04");//初始日期

}catch(Exception e){

    }   
    c.setTime(date);//设置日历时间   
    c.add(Calendar.MONTH,month);//在日历的月份上增加6个月   
    String strDate = sdf.format(c.getTime()));//的到你想要得6个月后的日期   
    return strDate;

}

/**

得到二个日期间的间隔天数 / public static String getTwoDay(String sj1, String sj2) { SimpleDateFormat myFormatter = new SimpleDateFormat("yyyy-MM-dd"); long day = 0; try { java.util.Date date = myFormatter.parse(sj1); java.util.Date mydate = myFormatter.parse(sj2); day = (date.getTime() - mydate.getTime()) / (24 * 60 * 60 * 1000); } catch (Exception e) { return ""; } return day + ""; } /*
根据一个日期，返回是星期几的字符串 / public static String getWeek(String sdate) { // 再转换为时间 Date date = TimeTest.strToDate(sdate); Calendar c = Calendar.getInstance(); c.setTime(date); // int hour=c.get(Calendar.DAY_OF_WEEK); // hour 中存的就是星期几了，其范围 1~7 // 1=星期日 7=星期六，其他类推 return new SimpleDateFormat("EEEE").format(c.getTime()); } /*
将短时间格式字符串转换为时间 yyyy-MM-dd / public static Date strToDate(String strDate) { SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd"); ParsePosition pos = new ParsePosition(0); Date strtodate = formatter.parse(strDate, pos); return strtodate; } /*
两个时间之间的天数 *
@param date1
@param date2
@return */ public static long getDays(String date1, String date2) { if (date1 == null || date1.equals("")) return 0; if (date2 == null || date2.equals("")) return 0; // 转换为标准时间 SimpleDateFormat myFormatter = new SimpleDateFormat("yyyy-MM-dd"); java.util.Date date = null; java.util.Date mydate = null; try { date = myFormatter.parse(date1); mydate = myFormatter.parse(date2); } catch (Exception e) { } long day = (date.getTime() - mydate.getTime()) / (24 * 60 * 60 * 1000); return day; } // 计算当月最后一天,返回字符串 public String getDefaultDay(){ String str = ""; SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd"); Calendar lastDate = Calendar.getInstance(); lastDate.set(Calendar.DATE,1);//设为当前月的1 号 lastDate.add(Calendar.MONTH,1);//加一个月，变为下月的1 号 lastDate.add(Calendar.DATE,-1);//减去一天，变为当月最后一天 str=sdf.format(lastDate.getTime()); return str; } // 上月第一天 public String getPreviousMonthFirst(){ String str = ""; SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd"); Calendar lastDate = Calendar.getInstance(); lastDate.set(Calendar.DATE,1);//设为当前月的1 号 lastDate.add(Calendar.MONTH,-1);//减一个月，变为下月的1 号 //lastDate.add(Calendar.DATE,-1);//减去一天，变为当月最后一天 str=sdf.format(lastDate.getTime()); return str; } //获取当月第一天 public String getFirstDayOfMonth(){ String str = ""; SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd"); Calendar lastDate = Calendar.getInstance(); lastDate.set(Calendar.DATE,1);//设为当前月的1 号 str=sdf.format(lastDate.getTime()); return str; } // 获得本周星期日的日期 public String getCurrentWeekday() { weeks = 0; int mondayPlus = this.getMondayPlus(); GregorianCalendar currentDate = new GregorianCalendar(); currentDate.add(GregorianCalendar.DATE, mondayPlus+6); Date monday = currentDate.getTime(); DateFormat df = DateFormat.getDateInstance(); String preMonday = df.format(monday); return preMonday; } //获取当天时间 public String getNowTime(String dateformat){ Date now = new Date(); SimpleDateFormat dateFormat = new SimpleDateFormat(dateformat);//可以方便地修改日期格式 String hehe = dateFormat.format(now); return hehe; } // 获得当前日期与本周日相差的天数 private int getMondayPlus() { Calendar cd = Calendar.getInstance(); // 获得今天是一周的第几天，星期日是第一天，星期二是第二天...... int dayOfWeek = cd.get(Calendar.DAY_OF_WEEK)-1; //因为按中国礼拜一作为第一天所以这里减1 if (dayOfWeek == 1) { return 0; } else { return 1 - dayOfWeek; } } //获得本周一的日期 public String getMondayOFWeek(){ weeks = 0; int mondayPlus = this.getMondayPlus(); GregorianCalendar currentDate = new GregorianCalendar(); currentDate.add(GregorianCalendar.DATE, mondayPlus); Date monday = currentDate.getTime(); DateFormat df = DateFormat.getDateInstance(); String preMonday = df.format(monday); return preMonday; } //获得相应周的周六的日期 public String getSaturday() { int mondayPlus = this.getMondayPlus(); GregorianCalendar currentDate = new GregorianCalendar(); currentDate.add(GregorianCalendar.DATE, mondayPlus + 7 * weeks + 6); Date monday = currentDate.getTime(); DateFormat df = DateFormat.getDateInstance(); String preMonday = df.format(monday); return preMonday; } // 获得上周星期日的日期 public String getPreviousWeekSunday() { weeks=0; weeks--; int mondayPlus = this.getMondayPlus(); GregorianCalendar currentDate = new GregorianCalendar(); currentDate.add(GregorianCalendar.DATE, mondayPlus+weeks); Date monday = currentDate.getTime(); DateFormat df = DateFormat.getDateInstance(); String preMonday = df.format(monday); return preMonday; } // 获得上周星期一的日期 public String getPreviousWeekday() { weeks--; int mondayPlus = this.getMondayPlus(); GregorianCalendar currentDate = new GregorianCalendar(); currentDate.add(GregorianCalendar.DATE, mondayPlus + 7 * weeks); Date monday = currentDate.getTime(); DateFormat df = DateFormat.getDateInstance(); String preMonday = df.format(monday); return preMonday; } // 获得下周星期一的日期 public String getNextMonday() { weeks++; int mondayPlus = this.getMondayPlus(); GregorianCalendar currentDate = new GregorianCalendar(); currentDate.add(GregorianCalendar.DATE, mondayPlus + 7); Date monday = currentDate.getTime(); DateFormat df = DateFormat.getDateInstance(); String preMonday = df.format(monday); return preMonday; } // 获得下周星期日的日期 public String getNextSunday() { int mondayPlus = this.getMondayPlus(); GregorianCalendar currentDate = new GregorianCalendar(); currentDate.add(GregorianCalendar.DATE, mondayPlus + 7+6); Date monday = currentDate.getTime(); DateFormat df = DateFormat.getDateInstance(); String preMonday = df.format(monday); return preMonday; } private int getMonthPlus(){ Calendar cd = Calendar.getInstance(); int monthOfNumber = cd.get(Calendar.DAY_OF_MONTH); cd.set(Calendar.DATE, 1);//把日期设置为当月第一天 cd.roll(Calendar.DATE, -1);//日期回滚一天，也就是最后一天 MaxDate=cd.get(Calendar.DATE); if(monthOfNumber == 1){ return -MaxDate; }else{ return 1-monthOfNumber; } } //获得上月最后一天的日期 public String getPreviousMonthEnd(){ String str = ""; SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd"); Calendar lastDate = Calendar.getInstance(); lastDate.add(Calendar.MONTH,-1);//减一个月 lastDate.set(Calendar.DATE, 1);//把日期设置为当月第一天 lastDate.roll(Calendar.DATE, -1);//日期回滚一天，也就是本月最后一天 str=sdf.format(lastDate.getTime()); return str; } //获得下个月第一天的日期 public String getNextMonthFirst(){ String str = ""; SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd"); Calendar lastDate = Calendar.getInstance(); lastDate.add(Calendar.MONTH,1);//减一个月 lastDate.set(Calendar.DATE, 1);//把日期设置为当月第一天 str=sdf.format(lastDate.getTime()); return str; } //获得下个月最后一天的日期 public String getNextMonthEnd(){ String str = ""; SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd"); Calendar lastDate = Calendar.getInstance(); lastDate.add(Calendar.MONTH,1);//加一个月 lastDate.set(Calendar.DATE, 1);//把日期设置为当月第一天 lastDate.roll(Calendar.DATE, -1);//日期回滚一天，也就是本月最后一天 str=sdf.format(lastDate.getTime()); return str; } //获得明年最后一天的日期 public String getNextYearEnd(){ String str = ""; SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd"); Calendar lastDate = Calendar.getInstance(); lastDate.add(Calendar.YEAR,1);//加一个年 lastDate.set(Calendar.DAY_OF_YEAR, 1); lastDate.roll(Calendar.DAY_OF_YEAR, -1); str=sdf.format(lastDate.getTime()); return str; } //获得明年第一天的日期 public String getNextYearFirst(){ String str = ""; SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd"); Calendar lastDate = Calendar.getInstance(); lastDate.add(Calendar.YEAR,1);//加一个年 lastDate.set(Calendar.DAY_OF_YEAR, 1); str=sdf.format(lastDate.getTime()); return str; } //获得本年有多少天 private int getMaxYear(){ Calendar cd = Calendar.getInstance(); cd.set(Calendar.DAY_OF_YEAR,1);//把日期设为当年第一天 cd.roll(Calendar.DAY_OF_YEAR,-1);//把日期回滚一天。 int MaxYear = cd.get(Calendar.DAY_OF_YEAR); return MaxYear; } private int getYearPlus(){ Calendar cd = Calendar.getInstance(); int yearOfNumber = cd.get(Calendar.DAY_OF_YEAR);//获得当天是一年中的第几天 cd.set(Calendar.DAY_OF_YEAR,1);//把日期设为当年第一天 cd.roll(Calendar.DAY_OF_YEAR,-1);//把日期回滚一天。 int MaxYear = cd.get(Calendar.DAY_OF_YEAR); if(yearOfNumber == 1){ return -MaxYear; }else{ return 1-yearOfNumber; } } //获得本年第一天的日期 public String getCurrentYearFirst(){ int yearPlus = this.getYearPlus(); GregorianCalendar currentDate = new GregorianCalendar(); currentDate.add(GregorianCalendar.DATE,yearPlus); Date yearDay = currentDate.getTime(); DateFormat df = DateFormat.getDateInstance(); String preYearDay = df.format(yearDay); return preYearDay; } //获得本年最后一天的日期 * public String getCurrentYearEnd(){ Date date = new Date(); SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy");//可以方便地修改日期格式 String years = dateFormat.format(date); return years+"-12-31"; }

//获得上年第一天的日期 *
public String getPreviousYearFirst(){
Date date = new Date();
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy");//可以方
便地修
改日期格式
String years = dateFormat.format(date); int years_value = Integer.parseInt
(years);
years_value--;
return years_value+"-1-1";
}
//获得上年最后一天的日期
public String getPreviousYearEnd(){
weeks--;
int yearPlus = this.getYearPlus();
GregorianCalendar currentDate = new GregorianCalendar();
currentDate.add(GregorianCalendar.DATE,yearPlus+MaxYear*weeks+(MaxYear-1));
Date yearDay = currentDate.getTime();
DateFormat df = DateFormat.getDateInstance();
String preYearDay = df.format(yearDay);
getThisSeasonTime(11);
return preYearDay;
}
//获得本季度
public String getThisSeasonTime(int month){
int array[][] = {{1,2,3},{4,5,6},{7,8,9},{10,11,12}};
int season = 1;
if(month>=1&&month<=3){
season = 1;
}
if(month>=4&&month<=6){
season = 2;
}
if(month>=7&&month<=9){
season = 3;
}
if(month>=10&&month<=12){
season = 4;
}
int start_month = array[season-1][0];
int end_month = array[season-1][2];
Date date = new Date();
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy");//可以方
便地修
改日期格式
String years = dateFormat.format(date);
int years_value = Integer.parseInt(years);
int start_days =1;//years+"-"+String.valueOf(start_month)+"-
1";//getLastDayOfMonth(years_value,start_month);
int end_days = getLastDayOfMonth(years_value,end_month);
String seasonDate =
years_value+"-"+start_month+"-"+start_days+";"+years_value+"-"+end_month+"-"+end_days;
return seasonDate;
}
/**

获取某年某月的最后一天
@param year 年
@param month 月
@return 最后一天 */ private int getLastDayOfMonth(int year, int month) { if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) { return 31; } if (month == 4 || month == 6 || month == 9 || month == 11) { return 30; } if (month == 2) { if (isLeapYear(year)) { return 29; } else { return 28; } } return 0; }

/**

是否闰年
@param year 年
@return */ public boolean isLeapYear(int year) { return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0); }

我用oracle数据库可以，java的没弄过
在oracle数据库中：
select round(to_date('13-2月-03'),'YEAR') from dual
select round(to_date('19-2月-03'),'month') from dual;
select round(to_date('11-9月-13'),'day') from dual;

SimpleDateFormat format = new SimpleDateFormat( "yyyy-MM-dd HH:mm:ss" );
if(birthday!=null && !"".equals(birthday)){
Date birthdaydate = format.parse(birthday);

Public string MonthAdd (string dates) {
SimpleDateFormat SDF = new SimpleDateFormat (" yyyyMMdd ");
Date dt = SDF. Parse (dates);
Calendar rightNow = Calendar. GetInstance ();
RightNow. SetTime (dt);
RightNow. Add (Calendar MONTH, 9);/ / date plus 9 months
Date newdate = rightNow. GetTime ();
String Str = SDF. The format (newdate);
Return the Str;
}