一个工具类，可以得到指定月份的天数，可以得到以前或以后多少天、周、月、年的日期等。

代码如下：

import android.annotation.SuppressLint;
import android.text.TextUtils;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

@SuppressLint("SimpleDateFormat")
public class DateHelper {

    private static DateHelper util;

    public static DateHelper getInstance() {

        if (util == null) {
            util = new DateHelper();
        }
        return util;

    }
    private DateHelper() {
        super();
    }

    public SimpleDateFormat date_Formater_1 = new SimpleDateFormat(
            "yyyy-MM-dd HH:mm:ss");


    public SimpleDateFormat date_Formater_2 = new SimpleDateFormat("yyyy-MM-dd");

    public Date getDate(String dateStr) {
        Date date = new Date();
        if (TextUtils.isEmpty(dateStr)) {
            return date;
        }
        try {
            date = date_Formater_1.parse(dateStr);
            return date;
        } catch (ParseException e) {
            e.printStackTrace();

        }
        return date;

    }

    public String getDataString_1(Date date) {
        if (date == null) {
            date = new Date();
        }
        String str = date_Formater_1.format(date);
        return str;

    }

    public String getDataString_2(Date date) {
        if (date == null) {
            date = new Date();
        }
        String str = date_Formater_2.format(date);
        return str;

    }

    /**
     * 将日期变成常见中文格式
     * 
     * @param date
     * @return
     */
    public String getRencentTime(String date) {
        Date time = getDate(date);
        if (time == null) {
            return "一个月前";
        }
        String ftime = "";
        Calendar cal = Calendar.getInstance();

        String curDate = date_Formater_2.format(cal.getTime());
        String paramDate = date_Formater_2.format(time);
        if (curDate.equals(paramDate)) {
            int hour = (int) ((cal.getTimeInMillis() - time.getTime()) / 3600000);
            if (hour == 0)
                ftime = Math.max(
                        (cal.getTimeInMillis() - time.getTime()) / 60000, 1)
                        + "分钟前";
            else
                ftime = hour + "小时前";
            return ftime;
        }

        long lt = time.getTime() / 86400000;
        long ct = cal.getTimeInMillis() / 86400000;
        int days = (int) (ct - lt);
        if (days == 0) {
            int hour = (int) ((cal.getTimeInMillis() - time.getTime()) / 3600000);
            if (hour == 0)
                ftime = Math.max(
                        (cal.getTimeInMillis() - time.getTime()) / 60000, 1)
                        + "分钟前";
            else
                ftime = hour + "小时前";
        } else if (days == 1) {
            ftime = "昨天";
        } else if (days == 2) {
            ftime = "前天";
        } else if (days > 2 && days <= 10) {
            ftime = days + "天前";
        } else if (days > 10) {
            ftime = "一个月前";
        } else {
            ftime = date_Formater_2.format(time);
        }
        return ftime;
    }

    /**
     * 日期时间格式转换
     * 
     * @param typeFrom
     *            原格式
     * @param typeTo
     *            转为格式
     * @param value
     *            传入的要转换的参数
     * @return
     */
    public String stringDateToStringData(String typeFrom, String typeTo,
            String value) {
        String re = value;
        SimpleDateFormat sdfFrom = new SimpleDateFormat(typeFrom);
        SimpleDateFormat sdfTo = new SimpleDateFormat(typeTo);

        try {
            re = sdfTo.format(sdfFrom.parse(re));
        } catch (ParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return re;
    }

    /**
     * 得到这个月有多少天
     * 
     * @param year
     * @param month
     * @return
     */
    public int getMonthLastDay(int year, int month) {
        if (month == 0) {
            return 0;
        }
        Calendar a = Calendar.getInstance();
        a.set(Calendar.YEAR, year);
        a.set(Calendar.MONTH, month - 1);
        a.set(Calendar.DATE, 1);// 把日期设置为当月第一天
        a.roll(Calendar.DATE, -1);// 日期回滚一天，也就是最后一天
        int maxDate = a.get(Calendar.DATE);
        return maxDate;
    }

    /**
     * 得到年份
     * 
     * @return
     */
    public String getCurrentYear() {
        Calendar c = Calendar.getInstance();
        return c.get(Calendar.YEAR) + "";
    }

    /**
     * 得到月份
     * 
     * @return
     */
    public String getCurrentMonth() {
        Calendar c = Calendar.getInstance();
        return (c.get(Calendar.MONTH) + 1) + "";
    }

    /**
     * 获得当天的日期
     * 
     * @return
     */
    public String getCurrDay() {
        Calendar c = Calendar.getInstance();
        return c.get(Calendar.DAY_OF_MONTH) + "";
    }

    /**
     * 得到几天/周/月/年后的日期，整数往后推,负数往前移动
     * 
     * @param calendar
     * @param calendarType
     *            Calendar.DATE,Calendar.WEEK_OF_YEAR,Calendar.MONTH,Calendar.
     *            YEAR
     * @param next
     * @return
     */
    public String getDayByDate(Calendar calendar, int calendarType, int next) {

        calendar.add(calendarType, next);
        Date date = calendar.getTime();
        String dateString = date_Formater_1.format(date);
        return dateString;

    }


}

使用方法

String dataStr = DateHelper.getInstance().getDataString_1(null);

        String toStringData = DateHelper.getInstance().stringDateToStringData("yyyy-MM-dd HH:mm:ss", "yyyy-MM-dd", dataStr);
        String date = DateHelper.getInstance().getDayByDate(
                Calendar.getInstance(), Calendar.DATE, 1);
        String week = DateHelper.getInstance().getDayByDate(
                Calendar.getInstance(), Calendar.WEEK_OF_YEAR, 1);
        String month = DateHelper.getInstance().getDayByDate(
                Calendar.getInstance(), Calendar.MONTH, 1);
        String year = DateHelper.getInstance().getDayByDate(
                Calendar.getInstance(), Calendar.YEAR, 1);
        int lastDay = DateHelper.getInstance().getMonthLastDay(2015, 2);
        System.out.println(dataStr);
        System.out.println(toStringData);
        System.out.println(date);
        System.out.println(week);
        System.out.println(month);
        System.out.println(year);
        System.out.println("2月有"+lastDay+"天");