SimpleDateFormat格式化

Date date = new Date();
/*注意，dd需要小写，另外，需要注意的是，匹配符字母不能随意写*/
/*获取date*/
SimpleDateFormat sdf1 = new SimpleDateFormat("YYYY年MM月dd日 ");
SimpleDateFormat sdf2 = new SimpleDateFormat("hh:mm:ss");
System.out.println(date);
System.out.println(sdf1.format(date));
System.out.println(sdf2.format(date));

Date

Date date = new Date();
//年份
System.out.println(date.getYear()+1900);
//月份
System.out.println(date.getMonth()+1);
//日期
System.out.println(date.getDate());
//星期数
 System.out.println(date.getDay());
//时
 System.out.println(date.getHours());
 //分
System.out.println(date.getMinutes());
//秒
System.out.println(date.getSeconds());
//毫秒值时间戳
System.out.println(date.getTime());

Calendar

Date date = new Date();

Calendar cal = Calendar.getInstance();
cal.setTime(date);

//获得年数
int year = cal.get(Calendar.YEAR);
//获得月数(比实际少一个月)
int month = cal.get(Calendar.MONTH);
//获得这一天在是这个年的第多少天
int day1 = cal.get(Calendar.DAY_OF_YEAR);
//获得这一天在是这个月的第多少天
int day2 = cal.get(Calendar.DAY_OF_MONTH);
//获得这一天在是这个周的第多少天(比实际多一天，国外周天是第一天)
int day3 = cal.get(Calendar.DAY_OF_WEEK);

求相差天数

方式一：通过Calendar类的日期比较

public static int differentDays(Date date1, Date date2) {
    Calendar cal1 = Calendar.getInstance();
    cal1.setTime(date1);

    Calendar cal2 = Calendar.getInstance();
    cal2.setTime(date2);
    int day1 = cal1.get(Calendar.DAY_OF_YEAR);
    int day2 = cal2.get(Calendar.DAY_OF_YEAR);

    int year1 = cal1.get(Calendar.YEAR);
    int year2 = cal2.get(Calendar.YEAR);
    if (year1 != year2){ // 不同年
        int timeDistance = 0;
        for (int i = year1; i < year2; i++) {
            if (i % 4 == 0 && i % 100 != 0 || i % 400 == 0){ // 闰年
                timeDistance += 366;
            } else {// 不是闰年
                timeDistance += 365;
            }
        }
        return timeDistance + (day2 - day1);
    } else {// 同一年
        return day2 - day1;
    }
}

方式二：直接用毫秒值计算

public static int differentDaysByMillisecond(Date date1,Date date2)
    {
        int days = (int) ((date2.getTime() - date1.getTime()) / (1000*3600*24));
        return days;
    }