在用POI的事件模型eventusermodel讀取大容量Excel時發現讀取的日期、時間格式輸出後是浮點數字符串，然而沒有找到有類似於usermodel的分格式讀取的方法，於是編寫一個工具類，把獲取的浮點數轉換成日期/時間格式，增加可用性。
Excel用浮點數標記日期時間，整數位代表日期，小數位代表時間。
程式碼如下：

package Util;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.GregorianCalendar;

/**
 * Created by yijie on 9/15/16.
 */
 

public class StringToDate {
    private static final int SECONDS_PER_MINUTE = 60;
    private static final int MINUTES_PER_HOUR = 60;
    private static final int HOURS_PER_DAY = 24;
    private static final int SECONDS_PER_DAY = (HOURS_PER_DAY * MINUTES_PER_HOUR * SECONDS_PER_MINUTE);
    /**
        一天的毫秒數
    **/
 

    private static final long DAY_MILLISECONDS = SECONDS_PER_DAY * 1000L;

    private static SimpleDateFormat sdFormat;

    /**
        轉換方法
        @parma numberString 要轉換的浮點數
        @parma format 要獲得的格式 例如"hh:mm:ss"
    **/
    public static String toDate(double numberString, String format) {
        sdFormat = new
 
 SimpleDateFormat(format);
        int wholeDays = (int)Math.floor(numberString);
        int millisecondsInday = (int)((numberString - wholeDays) * DAY_MILLISECONDS + 0.5);
        Calendar calendar = new GregorianCalendar();
        setCalendar(calendar, wholeDays, millisecondsInday, false);
        return sdFormat.format(calendar.getTime());

    }
    private static void setCalendar(Calendar calendar, int wholeDays,
                                   int millisecondsInDay, boolean use1904windowing) {
        int startYear = 1900;
        int dayAdjust = -1; // Excel thinks 2/29/1900 is a valid date, which it isn't
        if (use1904windowing) {
            startYear = 1904;
            dayAdjust = 1; // 1904 date windowing uses 1/2/1904 as the first day
        }
        else if (wholeDays < 61) {
            // Date is prior to 3/1/1900, so adjust because Excel thinks 2/29/1900 exists
            // If Excel date == 2/29/1900, will become 3/1/1900 in Java representation
            dayAdjust = 0;
        }
        calendar.set(startYear,0, wholeDays + dayAdjust, 0, 0, 0);
        calendar.set(GregorianCalendar.MILLISECOND, millisecondsInDay);
    }
}