• For greater efficiency, use Julian day number
• Used in astronomy
• Number of days since Jan. 1, 4713 BCE
• May 23, 1968 = Julian Day 2,440,000
• Greatly simplifies date arithmetic

 file:horstmann/ch03_day2/Day.java [source] [doc-public] [doc-private]
 ``` 001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 039 040 041 042 043 044 045 046 047 048 049 050 051 052 053 054 055 056 057 058 059 060 061 062 063 064 065 066 067 068 069 070 071 072 073 074 075 076 077 078 079 080 081 082 083 084 085 086 087 088 089 090 091 092 093 094 095 096 097 098 099 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 ``` ```package horstmann.ch03_day2; 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) { julian = toJulian(aYear, aMonth, aDate); } /** Returns the year of this day @return the year */ public int getYear() { return fromJulian(julian)[0]; } /** Returns the month of this day @return the month */ public int getMonth() { return fromJulian(julian)[1]; } /** Returns the day of the month of this day @return the day of the month */ public int getDate() { return fromJulian(julian)[2]; } /** 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) { 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) { return julian - other.julian; } private Day(int aJulian) { julian = aJulian; } /** Computes the Julian day number of the given 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 was 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. */ @SuppressWarnings("cast") 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 julian; } ```