1、获取系统当前日期

Date now = new Date();
Calendar calendar = Calendar.getInstance();
calendar.setTime(now);
calendar.add(calendar.MONTH, -0);
String day=new SimpleDateFormat("yyyy-MM-dd").format(calendar.getTime()); //设置时间格式
System.out.println(day);

2、获取当月第一天

Date now = new Date();
Calendar calendar = Calendar.getInstance();
calendar.setTime(now);
calendar.add(calendar.MONTH, -0);
// 设置日历中月份的最小天数
calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMinimum(Calendar.DAY_OF_MONTH));
String firstDay = new SimpleDateFormat("yyyy-MM-dd").format(calendar.getTime());//设置时间格式
System.out.println(firstDay);

3、获取上个月第一天

Date now = new Date();
Calendar calendar = Calendar.getInstance();
calendar.setTime(now);
calendar.add(calendar.MONTH, -1);//月份往前推1个月
// 设置日历中月份的最小天数
calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMinimum(Calendar.DAY_OF_MONTH));
String time = new SimpleDateFormat("yyyy-MM-dd").format(calendar.getTime());//设置时间格式
System.out.println(time);

4、获取半年前的今天的日期

Date now = new Date();
Calendar calendar = Calendar.getInstance();
calendar.setTime(now);
calendar.add(calendar.MONTH, -6);//月份往前推6个月(需要几个月前的当前天往前推几个月)
String time = new SimpleDateFormat("yyyy-MM-dd").format(calendar.getTime());//设置时间格式
System.out.println(time);

5、获取本月最后一天日期

Date dNow = new Date();
Calendar calendar = Calendar.getInstance();
calendar.setTime(dNow);
calendar.add(calendar.MONTH, 0);
// 获取某月最大天数
int lastDay=0;
//2月的平年瑞年天数
if(Calendar.MONTH==1) {
    lastDay = calendar.getLeastMaximum(Calendar.DAY_OF_MONTH);
}else {
    lastDay = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
}
// 设置日历中月份的最大天数
calendar.set(Calendar.DAY_OF_MONTH, lastDay);
//设置时间格式
String time = new SimpleDateFormat("yyyy-MM-dd").format(calendar.getTime());
System.out.println(time);

6、获取上月最后一天日期

Date dNow = new Date();
Calendar calendar = Calendar.getInstance();
calendar.setTime(dNow);
calendar.add(calendar.MONTH, -1);
// 获取某月最大天数
int lastDay=0;
//2月的平年瑞年天数
if(Calendar.MONTH==1) {
    lastDay = calendar.getLeastMaximum(Calendar.DAY_OF_MONTH);
}else {
    lastDay = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
}
// 设置日历中月份的最大天数
calendar.set(Calendar.DAY_OF_MONTH, lastDay);
//设置时间格式
String time = new SimpleDateFormat("yyyy-MM-dd").format(calendar.getTime());
System.out.println(time);

7、获取今年第一天

Calendar calendar = Calendar.getInstance();
Calendar date = Calendar.getInstance();
calendar.clear();
calendar.set(Calendar.YEAR, Integer.parseInt(String.valueOf(date.get(Calendar.YEAR))));
String time = new SimpleDateFormat("yyyy-MM-dd").format(calendar.getTime());
System.out.println(time);

8、获取去年第一天

Calendar calendar = Calendar.getInstance();
Calendar date = Calendar.getInstance();
calendar.clear();
calendar.set(Calendar.YEAR, Integer.parseInt(String.valueOf(date.get(Calendar.YEAR)))-1);
String time = new SimpleDateFormat("yyyy-MM-dd").format(calendar.getTime());
System.out.println(time);

9、获取今年最后一天

Calendar calendar = Calendar.getInstance();
Calendar date = Calendar.getInstance();
calendar.clear();
calendar.set(Calendar.YEAR, Integer.parseInt(String.valueOf(date.get(Calendar.YEAR))));
calendar.roll(Calendar.DAY_OF_YEAR, -1);
String time = new SimpleDateFormat("yyyy-MM-dd").format(calendar.getTime());
System.out.println(time);

10、获取去年最后一天

Calendar calendar = Calendar.getInstance();
Calendar date = Calendar.getInstance();
calendar.clear();
calendar.set(Calendar.YEAR, Integer.parseInt(String.valueOf(date.get(Calendar.YEAR)))-1);
calendar.roll(Calendar.DAY_OF_YEAR, -1);
String time = new SimpleDateFormat("yyyy-MM-dd").format(calendar.getTime());
System.out.println(time);

11、获取7天前的日期

Calendar calendar = new GregorianCalendar();
calendar.setTime(new Date());
calendar.add(calendar.DATE, -7);
String time = new SimpleDateFormat("yyyy-MM-dd").format(calendar.getTime());
System.out.println(time);

12、获取int类型的年份

//上一年
int years = Integer.parseInt(String.valueOf(Calendar.getInstance().get(Calendar.YEAR)))-1;
System.out.println(years);
//当前年
int year= Integer.parseInt(String.valueOf(Calendar.getInstance().get(Calendar.YEAR)));
System.out.println(year);
//下一年
int yearss = Integer.parseInt(String.valueOf(Calendar.getInstance().get(Calendar.YEAR)))+1;
System.out.println(yearss);

13、获取int类型的天

//后一天
int days = Integer.parseInt(String.valueOf(Calendar.getInstance().get(Calendar.DAY_OF_MONTH)+1));
System.out.println(days);
//当前天
int day = Integer.parseInt(String.valueOf(Calendar.getInstance().get(Calendar.DAY_OF_MONTH)));
System.out.println(day);
//前一天
int dayss = Integer.parseInt(String.valueOf(Calendar.getInstance().get(Calendar.DAY_OF_MONTH)-1));
System.out.println(dayss);

14、获取int类型的月份

//当前月
int month = Integer.parseInt(String.valueOf(Calendar.getInstance().get(Calendar.MONTH)+1));
System.out.println(month);
//上个月
int months = Integer.parseInt(String.valueOf(Calendar.getInstance().get(Calendar.MONTH)));
System.out.println(months);
//下个月
int monthss = Integer.parseInt(String.valueOf(Calendar.getInstance().get(Calendar.MONTH)+2));
System.out.println(monthss);