public class Day
{
   /**
      Constructs a day with a given year, month, and day
      of the Julian/Gregorian calendar. The Julian calendar
      is used for all days before October 15, 1582
      @param aYear a year != 0
      @param aMonth a month between 1 and 12
      @param aDate a date between 1 and 31
   */
   public Day(int aYear, int aMonth, int aDate)
   {
      year = aYear;
      month = aMonth;
      date = aDate;
      ymdValid = true;
      julianValid = false;
   }

   /**
      Returns the year of this day
      @return the year
   */
   public int getYear()
   {
      ensureYmd();
      return year;
   }

   /**
      Returns the month of this day
      @return the month
   */
   public int getMonth()
   {
      ensureYmd();
      return month;
   }

   /**
      Returns the day of the month of this day
      @return the day of the month
   */
   public int getDate()
   {
      ensureYmd();
      return date;
   }

   /**
      Returns a day that is a certain number of days away from
      this day
      @param n the number of days, can be negative
      @return a day that is n days away from this one
   */
   public Day addDays(int n)
   {
      ensureJulian();
      return new Day(julian + n);
   }

   /**
      Returns the number of days between this day and another
      day
      @param other the other day
      @return the number of days that this day is away from 
      the other (>0 if this day comes later)
   */
   public int daysFrom(Day other)
   {
      ensureJulian();
      other.ensureJulian();
      return julian - other.julian;
   }

   private Day(int aJulian)
   {
      julian = aJulian;
      ymdValid = false;
      julianValid = true;
   }

   /**
      Computes the Julian day number of this day if 
      necessary
   */
   private void ensureJulian()
   {  
      if (julianValid) return;
      julian = toJulian(year, month, date);
      julianValid = true;
   }

   /**
      Converts this Julian day mumber to a calendar date if necessary.
   */
   private void ensureYmd()
   {  
      if (ymdValid) return;
      int[] ymd = fromJulian(julian);
      year = ymd[0];
      month = ymd[1];
      date = ymd[2];
      ymdValid = true;
   }

   /**
      Computes the Julian day number of the given day day.

      @param year a year
      @param month a month
      @param date a day of the month
      @return The Julian day number that begins at noon of 
      the given day
      Positive year signifies CE, negative year BCE. 
      Remember that the year after 1 BCE is 1 CE.

      A convenient reference point is that May 23, 1968 noon
      is Julian day number 2440000.

      Julian day number 0 is a Monday.

      This algorithm is from Press et al., Numerical Recipes
      in C, 2nd ed., Cambridge University Press 1992
   */
   private static int toJulian(int year, int month, int date)
   {  
      int jy = year;
      if (year < 0) jy++;
      int jm = month;
      if (month > 2) jm++;
      else
      {  
         jy--;
         jm += 13;
      }
      int jul = (int) (java.lang.Math.floor(365.25 * jy) 
            + java.lang.Math.floor(30.6001 * jm) + date + 1720995.0);

      int IGREG = 15 + 31 * (10 + 12 * 1582);
         // Gregorian Calendar adopted Oct. 15, 1582

      if (date + 31 * (month + 12 * year) >= IGREG)
         // Change over to Gregorian calendar
      {  
         int ja = (int) (0.01 * jy);
         jul += 2 - ja + (int) (0.25 * ja);
      }
      return jul;
   }

   /**
      Converts a Julian day number to a calendar date.
      
      This algorithm is from Press et al., Numerical Recipes
      in C, 2nd ed., Cambridge University Press 1992

      @param j  the Julian day number
      @return an array whose 0 entry is the year, 1 the month,
      and 2 the day of the month.
   */
   private static int[] fromJulian(int j)
   {  
      int ja = j;
   
      int JGREG = 2299161;
         // The Julian day number of the adoption of the Gregorian calendar    

      if (j >= JGREG)
         // Cross-over to Gregorian Calendar produces this correction
      {  
         int jalpha = (int) (((float) (j - 1867216) - 0.25) 
             / 36524.25);
         ja += 1 + jalpha - (int) (0.25 * jalpha);
      }
      int jb = ja + 1524;
      int jc = (int) (6680.0 + ((float) (jb - 2439870) - 122.1)
          / 365.25);
      int jd = (int) (365 * jc + (0.25 * jc));
      int je = (int) ((jb - jd) / 30.6001);
      int date = jb - jd - (int) (30.6001 * je);
      int month = je - 1;
      if (month > 12) month -= 12;
      int year = jc - 4715;
      if (month > 2) --year;
      if (year <= 0) --year;
      return new int[] { year, month, date };
   }

   private int year;
   private int month;
   private int date;
   private int julian;
   private boolean ymdValid;
   private boolean julianValid;
}