dates.java

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		cal.setTimeInMillis(cal.getTimeInMillis() - 1);		
		return cal.getTime();
	}

	/**
	 * Return the ending date of this year.
	 */
	final public static short twoMonthShort() {
		return twoMonthShort(new Date(), null);
	}
	/**
	 * Given a date, a proper TimeZone, return the two month int. eg. 1, 3, 5, 7, 9, 11.
	 * If TimeZone is null, meaning use default TimeZone of the JVM.
	 */
	final public static short twoMonthShort(Date when, TimeZone tz) {
		if (tz == null)
			tz = TimeZones.getCurrent();
		final Calendar cal = Calendar.getInstance(tz);
		cal.setTimeInMillis(when.getTime()); //don't call cal.setTime(Date) which will reset the TimeZone.

		final int month = (cal.get(Calendar.MONTH) / 2) * 2 + 1;
		return (short)month;
	}
	
	/**
	 * Get the year of a date.
	 * @param when The date.
	 * @param tz The time zone; if null, the current time zone is assumed.
	 * @see #localizedYearOfDate
	 */
	public static final int yearOfDate(Date when, TimeZone tz) {
		if (tz == null)
			tz = TimeZones.getCurrent();
		final Calendar cal = Calendar.getInstance(tz);
		cal.setTimeInMillis(when.getTime()); //don't call cal.setTime(Date) which will reset the TimeZone.
		
		return cal.get(Calendar.YEAR);
	}
	
	/**
	 * Get the year of a date in the specified locale.
	 *
	 * <p>Currenty, only Locale.ZH_TW is supported, i.e.,
	 * "year - 1911" and it's may be less than 0. Otherwise, it is the same
	 * as {@link #yearOfDate}.
	 *
	 * @param when The date.
	 * @param locale the locale; if null, the current locale is assumed.
	 * @param tz The time zone; if null, the current time zone is assumed.
	 * @see #yearOfDate
	 */
	public static final int
	localizedYearOfDate(Date when, Locale locale, TimeZone tz) {
		if (locale == null)
			locale = Locales.getCurrent();

		final int year = yearOfDate(when, tz);
		if (locale.equals(Locale.TAIWAN))
			return year - 1911;
		return year;
	}
	
	/**
	 * Get the month of a date. The first month of the year is JANUARY which is 0.
	 * @param when The date.
	 * @param tz The time zone; if null, the current time zone is assumed.
	 */
	public static final int monthOfDate(Date when, TimeZone tz) {
		if (tz == null)
			tz = TimeZones.getCurrent();
		final Calendar cal = Calendar.getInstance(tz);
		cal.setTimeInMillis(when.getTime()); //don't call cal.setTime(Date) which will reset the TimeZone.
		
		return cal.get(Calendar.MONTH);
	}
	
	/**
	 * Get the month of a date. The first month of the year is JANUARY which is 1.
	 * @param when The date.
	 * @param tz The time zone; if null, the current time zone is assumed.
	 */
	public static final int monthOfDatePlus1(Date when, TimeZone tz) {
		if (tz == null)
			tz = TimeZones.getCurrent();
		final Calendar cal = Calendar.getInstance(tz);
		cal.setTimeInMillis(when.getTime()); //don't call cal.setTime(Date) which will reset the TimeZone.
		
		return cal.get(Calendar.MONTH) + 1;
	}
	
	/**
	 * Get the day of month of a date. The first day of the month has value 1.
	 * @param when The date.
	 * @param tz The time zone; if null, the current time zone is assumed.
	 */
	public static final int dayMonthOfDate(Date when, TimeZone tz) {
		if (tz == null)
			tz = TimeZones.getCurrent();
		final Calendar cal = Calendar.getInstance(tz);
		cal.setTimeInMillis(when.getTime()); //don't call cal.setTime(Date) which will reset the TimeZone.
		
		return cal.get(Calendar.DAY_OF_MONTH);
	}
	
	/**
	 * Date Arithmetic function. Adds the specified (signed) amount of time to the given date, 
	 * based on the calendar's rules.
	 * @param when The based date.
	 * @param tz The time zone; if null, the current time zone is assumed.
	 * @param field The time field.
	 * @param amount The amount of date or time to be added to the field.
	 */
	public static final Date add(Date when, TimeZone tz, int field, int amount) {
		if (tz == null)
			tz = TimeZones.getCurrent();

		final Calendar cal = Calendar.getInstance(tz);
		cal.setTimeInMillis(when.getTime());//don't call cal.setTime(Date) which will reset the TimeZone.
		
		cal.add(field, amount);
		return cal.getTime();
	}

	/**
	 * Date Arithmetic function (date2 - date1). subtract a date from another date, return the
	 * difference as the required fields. E.g. if specified Calendar.Date, the
	 * smaller range of fields is ignored and this method return the difference
	 * of days.
	 *
	 * @param date2 The date2.
	 * @param tz The time zone.
	 * @param field The time field; e.g., Calendar.DATE, Calendar.YEAR, it's default value is Calendar.DATE
	 * @param date1 The date1.
	 */
	public static final long
	subtract(Date date2, TimeZone tz, int field, Date date1) {
		if (tz == null)
			tz = TimeZones.getCurrent();

		boolean negative = false;
		if (date1.after(date2)) {
			negative = true;
			final Date d = date1;
			date1 = date2;
			date2 = d;
		}
		
		final Calendar cal1 = Calendar.getInstance(tz);
		cal1.setTimeInMillis(date1.getTime());//don't call cal.setTime(Date) which will reset the TimeZone.

		final Calendar cal2 = Calendar.getInstance(tz);
		cal2.setTimeInMillis(date2.getTime());//don't call cal.setTime(Date) which will reset the TimeZone.
		
		int year1 = cal1.get(Calendar.YEAR);
		int year2 = cal2.get(Calendar.YEAR);
		
		switch(field) {
			case Calendar.YEAR:
			{
				return negative ? (year1-year2) : (year2-year1);
			}
			case Calendar.MONTH:
			{
				int month1 = cal1.get(Calendar.MONTH);
				int month2 = cal2.get(Calendar.MONTH);
				int months = 12 * (year2 - year1) + month2 - month1;
				return negative ? -months : months;
			}
			case Calendar.HOUR:
			{
				int hour1 = cal1.get(Calendar.HOUR_OF_DAY);
				int hour2 = cal2.get(Calendar.HOUR_OF_DAY);
				int day1 = cal1.get(Calendar.DAY_OF_YEAR);
				int day2 = cal2.get(Calendar.DAY_OF_YEAR);
				
				int maxDay1 = year1 == year2 ? 
					0 : cal1.getActualMaximum(Calendar.DAY_OF_YEAR);
				int hours = 24 * (maxDay1 - day1 + day2) + hour2 - hour1;
				
				final Calendar cal = Calendar.getInstance(tz);
				for(int year = year1+1; year < year2; year++) {
					cal.set(Calendar.YEAR, year);
					hours += 24 * cal.getActualMaximum(Calendar.DAY_OF_YEAR);
				}
				return negative ? -hours : hours;
			}
			case Calendar.MINUTE:
			{
				int minute1 = cal1.get(Calendar.MINUTE);
				int minute2 = cal2.get(Calendar.MINUTE);
				int hour1 = cal1.get(Calendar.HOUR_OF_DAY);
				int hour2 = cal2.get(Calendar.HOUR_OF_DAY);
				int day1 = cal1.get(Calendar.DAY_OF_YEAR);
				int day2 = cal2.get(Calendar.DAY_OF_YEAR);
				
				int maxHour1 = day1 == day2 ? 0 : 24;
				int maxDay1 = year1 == year2 ? 
					0 : cal1.getActualMaximum(Calendar.DAY_OF_YEAR);
	
				int minutes = 60 * (maxHour1 - hour1 + hour2) + minute2 - minute1
					+ 24 * 60 * (maxDay1 - day1 + day2);
				
				for(int year = year1+1; year < year2; year++) {
					final Calendar cal = Calendar.getInstance(tz);
					cal.set(Calendar.YEAR, year);
					minutes += 24 * 60 * cal.getActualMaximum(Calendar.DAY_OF_YEAR);
				}
				return negative ? -minutes : minutes;
			}
			case Calendar.SECOND:
			{
				long sec1 = date1.getTime() / 1000;
				long sec2 = date2.getTime() / 1000;
				return negative ? (sec1 - sec2) : (sec2 - sec1);
			}
			case Calendar.MILLISECOND:
			{
				return negative ? 
					 (date1.getTime() - date2.getTime()): 
					 (date2.getTime() - date1.getTime());
			}	
			case Calendar.DATE:
			default:	/*default, like -1*/
			{
				int day1 = cal1.get(Calendar.DAY_OF_YEAR);
				int day2 = cal2.get(Calendar.DAY_OF_YEAR);
	
				int maxDay1 = year1 == year2 ? 
					0 : cal1.getActualMaximum(Calendar.DAY_OF_YEAR);
				int days = maxDay1 - day1 + day2;
					
				final Calendar cal = Calendar.getInstance(tz);
				for(int year = year1+1; year < year2; year++) {
					cal.set(Calendar.YEAR, year);
					days += cal.getActualMaximum(Calendar.DAY_OF_YEAR);
				}
				return negative ? -days : days;
			}
		}
	}

	/**
	 * merge the date part and time part of two specified dates into a date.
	 * @param datePart The date part date.
	 * @param timePart The time part date.
	 * @param tz The time zone.
	 */
	public static final Date merge(Date datePart, Date timePart, TimeZone tz) {
		if (tz == null)
			tz = TimeZones.getCurrent();

		final Calendar dateCal = Calendar.getInstance(tz);
		dateCal.setTimeInMillis(datePart.getTime());//don't call cal.setTime(Date) which will reset the TimeZone.

		final Calendar timeCal = Calendar.getInstance(tz);
		timeCal.setTimeInMillis(timePart.getTime());//don't call cal.setTime(Date) which will reset the TimeZone.
		
		final int hour = timeCal.get(Calendar.HOUR);
		final int minute = timeCal.get(Calendar.MINUTE);
		final int second = timeCal.get(Calendar.SECOND);
		final int msillisecond = timeCal.get(Calendar.MILLISECOND);
		
		dateCal.set(Calendar.HOUR, hour);
		dateCal.set(Calendar.MINUTE, minute);
		dateCal.set(Calendar.SECOND, second);
		dateCal.set(Calendar.MILLISECOND, msillisecond);
		
		return dateCal.getTime();
	}
}