Get a julian date - Real's Java How-to

The starting point for the first Julian cycle began on January 1, 4713 B.C. and will end on January 22, 3268 (3268-01-22 G). The following day will begin the first day of the second Julian date period (or 7,980 year cycle).

import java.util.Calendar;
public class JulianDate {
 /**
  * Returns the Julian day number that begins at noon of
  * this day, Positive year signifies A.D., negative year B.C.
  * Remember that the year after 1 B.C. was 1 A.D.
  *
  * ref :
  *  Numerical Recipes in C, 2nd ed., Cambridge University Press 1992
  */
  // Gregorian Calendar adopted Oct. 15, 1582 (2299161)
  public static int JGREG= 15 + 31*(10+12*1582);
  public static double HALFSECOND = 0.5;

  public static double toJulian(int[] ymd) {
   int year=ymd[0];
   int month=ymd[1]; // jan=1, feb=2,...
   int day=ymd[2];
   int julianYear = year;
   if (year < 0) julianYear++;
   int julianMonth = month;
   if (month > 2) {
     julianMonth++;
   }
   else {
     julianYear--;
     julianMonth += 13;
   }

   double julian = (java.lang.Math.floor(365.25 * julianYear)
        + java.lang.Math.floor(30.6001*julianMonth) + day + 1720995.0);
   if (day + 31 * (month + 12 * year) >= JGREG) {
     // change over to Gregorian calendar
     int ja = (int)(0.01 * julianYear);
     julian += 2 - ja + (0.25 * ja);
   }
   return java.lang.Math.floor(julian);
 }

 /**
 * Converts a Julian day to a calendar date
 * ref :
 * Numerical Recipes in C, 2nd ed., Cambridge University Press 1992
 */
 public static int[] fromJulian(double injulian) {
   int jalpha,ja,jb,jc,jd,je,year,month,day;
   double julian = injulian + HALFSECOND / 86400.0;
   ja = (int) julian
   if (ja>= JGREG) {
     jalpha = (int) (((ja - 1867216) - 0.25) / 36524.25);
     ja = ja + 1 + jalpha - jalpha / 4;
   }

   jb = ja + 1524;
   jc = (int) (6680.0 + ((jb - 2439870) - 122.1) / 365.25);
   jd = 365 * jc + jc / 4;
   je = (int) ((jb - jd) / 30.6001);
   day = jb - jd - (int) (30.6001 * je);
   month = je - 1;
   if (month > 12) month = month - 12;
   year = jc - 4715;
   if (month > 2) year--;
   if (year <= 0) year--;

   return new int[] {year, month, day};
  }

  public static void main(String args[]) {
   // FIRST TEST reference point
   System.out.println("Julian date for May 23, 1968 : "
      + toJulian( new int[] {1968, 5, 23 } ));
   // output : 2440000
   int results[] = fromJulian(toJulian(new int[] {1968, 5, 23 }));
   System.out.println
     ("... back to calendar : " + results[0] + " "
       + results[1] + " " + results[2]);

   // SECOND TEST today
   Calendar today = Calendar.getInstance();
   double todayJulian = toJulian
      (new int[]{today.get(Calendar.YEAR), today.get(Calendar.MONTH)+1,
       today.get(Calendar.DATE)});
   System.out.println("Julian date for today : " + todayJulian);
   results = fromJulian(todayJulian);
   System.out.println
      ("... back to calendar : " + results[0] + " " + results[1]
       + " " + results[2]);

   // THIRD TEST
   double date1 = toJulian(new int[]{2005,1,1});
   double date2 = toJulian(new int[]{2005,1,31});
   System.out.println("Between 2005-01-01 and 2005-01-31 : "
      + (date2 - date1) + " days");

   /*
      expected output :
         Julian date for May 23, 1968 : 2440000.0
         ... back to calendar 1968 5 23
         Julian date for today : 2453487.0
         ... back to calendar 2005 4 26
         Between 2005-01-01 and 2005-01-31 : 30.0 days
   */
   }
 }

Before Y2K, many applications (especially mainframe systems) were storing dates in a format called "the Julian format". This format is a 5 digit number, consisting of a 2 digit year and a 3 digit day-of-year number. For example, 17-July-1998 is stored as 98221, since 17-July is the 221th day of the year. This format is not really useful since Y2K! The main reason for using the 5-digits Julian date was to save disk space and still have a format easy to use to handle. dates.

A variation of this idea is to used the first four digits for the year and 3 digits for day-of-year, 17-July-1998 will be represented by 1998221.

import java.util.Calendar;
import java.util.Date;
import java.text.ParseException;
import java.text.SimpleDateFormat;

public class DateUtils {
  private DateUtils() {  }

  public static Date getDateFromJulian7(String julianDate)
  throws ParseException
  {
    return new SimpleDateFormat("yyyyD").parse(julianDate);
  }

  public static String getJulian7FromDate(Date date) {
    StringBuilder sb = new StringBuilder();
    Calendar cal  = Calendar.getInstance();
    cal.setTime(date);

    return  sb.append(cal.get(Calendar.YEAR))
              .append(String.format("%03d", cal.get(Calendar.DAY_OF_YEAR)))
              .toString();
  }

  public static void main(String[] args) throws Exception {
    String test = "1998221";
    Date d = DateUtils.getDateFromJulian7(test);
    System.out.println(d);
    System.out.println(DateUtils.getJulian7FromDate(d));

    /*
     * output :
     *    Sun Aug 09 00:00:00 EDT 1998
     *    1998221
     */
  }
}

Get a julian dateTag(s): Date and Time