UTC(世界协调时间也就是格林尼治时间，和北京时间差了八个小时)时间格式 ISO 8601 日期-时间格式表示：yyyy-mm-ddThh:mm:ss.fffZ，其中 yyyy = 年，mm = 月，dd = 天，T 表示时间部分的开头，hh = 小时，mm = 分钟，ss = 秒，fff = 几分之几秒，而 Z 表示祖鲁语（另一种 UTC 表示方法）

        //当前本地时间Date  对应的  UTC时间String
        Date date = new Date();
        System.out.println("本地时间Date: " + date);
        TimeZone tz = TimeZone.getTimeZone("UTC");
        DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'");
        df.setTimeZone(tz);  //获取时区
        String nowAsISO = df.format(date);
        System.out.println("本地时间Date--->UTC时间String：" + nowAsISO);

        

        //UTC时间String   对应的   本地时间Date
        String utcString = "2019-08-29T10:13:24.725Z";
        System.out.println("UTC时间字符串：" +  utcString);
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'");
        sdf.setTimeZone(TimeZone.getTimeZone("UTC"));  //获取时区
        Date isoStringToDate = null;
        try {
            isoStringToDate = sdf.parse(utcString);
        } catch (ParseException e) {
            e.printStackTrace();
        }
        System.out.println("UTC时间String--->本地时间Date: " + isoStringToDate);

  2    Date

//当天开始日期
Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.HOUR_OF_DAY, 0);
calendar.set(Calendar.SECOND, 0);
calendar.set(Calendar.MINUTE, 0);
Date dayBegin = calendar.getTime();

//当天结束日期
calendar = Calendar.getInstance();
calendar.set(Calendar.HOUR_OF_DAY,23);
calendar.set(Calendar.MINUTE,59);
calendar.set(Calendar.SECOND,59);
calendar.set(Calendar.MILLISECOND,999);
Date dayEnd = calendar.getTime();

//当月开始日期
calendar = Calendar.getInstance();
calendar.set(Calendar.HOUR_OF_DAY, 0);
calendar.set(Calendar.SECOND, 0);
calendar.set(Calendar.MINUTE, 0);
calendar.set(Calendar.DAY_OF_MONTH, 1);
Date monthBegin = calendar.getTime();

 //当月结束日期                       
calendar = Calendar.getInstance();
calendar.add(Calendar.MONTH, 1);
calendar.set(Calendar.DAY_OF_MONTH, 0);
calendar.set(Calendar.HOUR_OF_DAY,23);
calendar.set(Calendar.SECOND,59);
calendar.set(Calendar.MILLISECOND,999);
Date monthEnd = calendar.getTime();