http://www.joda.org/joda-time/: Date and time library to replace JDK date handling (Joda.org) 
/*
 *  Copyright 2001-2014 Stephen Colebourne
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */
package org.joda.time.chrono;

import java.util.concurrent.ConcurrentHashMap;

import org.joda.time.Chronology;
import org.joda.time.DateTimeConstants;
import org.joda.time.DateTimeFieldType;
import org.joda.time.DateTimeZone;
import org.joda.time.IllegalFieldValueException;
import org.joda.time.field.SkipDateTimeField;


Implements a pure proleptic Julian calendar system, which defines every
fourth year as leap. This implementation follows the leap year rule
strictly, even for dates before 8 CE, where leap years were actually
irregular. In the Julian calendar, year zero does not exist: 1 BCE is
followed by 1 CE.


Although the Julian calendar did not exist before 45 BCE, this chronology
assumes it did, thus it is proleptic. This implementation also fixes the
start of the year at January 1.


JulianChronology is thread-safe and immutable.

Author:Guy Allard, Brian S O'Neill, Stephen Colebourne
See Also:
Since:1.0
/**
 * Implements a pure proleptic Julian calendar system, which defines every
 * fourth year as leap. This implementation follows the leap year rule
 * strictly, even for dates before 8 CE, where leap years were actually
 * irregular. In the Julian calendar, year zero does not exist: 1 BCE is
 * followed by 1 CE.
 * <p>
 * Although the Julian calendar did not exist before 45 BCE, this chronology
 * assumes it did, thus it is proleptic. This implementation also fixes the
 * start of the year at January 1.
 * <p>
 * JulianChronology is thread-safe and immutable.
 *
 * @see <a href="http://en.wikipedia.org/wiki/Julian_calendar">Wikipedia</a>
 * @see GregorianChronology
 * @see GJChronology
 *
 * @author Guy Allard
 * @author Brian S O'Neill
 * @author Stephen Colebourne
 * @since 1.0
 */
public final class JulianChronology extends BasicGJChronology {

    
Serialization lock 
/** Serialization lock */
    private static final long serialVersionUID = -8731039522547897247L;

    private static final long MILLIS_PER_YEAR =
        (long) (365.25 * DateTimeConstants.MILLIS_PER_DAY);

    private static final long MILLIS_PER_MONTH =
        (long) (365.25 * DateTimeConstants.MILLIS_PER_DAY / 12);

    
The lowest year that can be fully supported. 
/** The lowest year that can be fully supported. */
    private static final int MIN_YEAR = -292269054;

    
The highest year that can be fully supported. 
/** The highest year that can be fully supported. */
    private static final int MAX_YEAR = 292272992;

    
Singleton instance of a UTC JulianChronology 
/** Singleton instance of a UTC JulianChronology */
    private static final JulianChronology INSTANCE_UTC;

    
Cache of zone to chronology arrays 
/** Cache of zone to chronology arrays */
    private static final ConcurrentHashMap<DateTimeZone, JulianChronology[]> cCache = new ConcurrentHashMap<DateTimeZone, JulianChronology[]>();

    static {
        INSTANCE_UTC = getInstance(DateTimeZone.UTC);
    }

    static int adjustYearForSet(int year) {
        if (year <= 0) {
            if (year == 0) {
                throw new IllegalFieldValueException
                    (DateTimeFieldType.year(), Integer.valueOf(year), null, null);
            }
            year++;
        }
        return year;
    }

    
Gets an instance of the JulianChronology.
The time zone of the returned instance is UTC.

Returns:a singleton UTC instance of the chronology
/**
     * Gets an instance of the JulianChronology.
     * The time zone of the returned instance is UTC.
     * 
     * @return a singleton UTC instance of the chronology
     */
    public static JulianChronology getInstanceUTC() {
        return INSTANCE_UTC;
    }

    
Gets an instance of the JulianChronology in the default time zone.

Returns:a chronology in the default time zone
/**
     * Gets an instance of the JulianChronology in the default time zone.
     * 
     * @return a chronology in the default time zone
     */
    public static JulianChronology getInstance() {
        return getInstance(DateTimeZone.getDefault(), 4);
    }

    
Gets an instance of the JulianChronology in the given time zone.

Params:
  • zone –  the time zone to get the chronology in, null is default
Returns:a chronology in the specified time zone
/**
     * Gets an instance of the JulianChronology in the given time zone.
     * 
     * @param zone  the time zone to get the chronology in, null is default
     * @return a chronology in the specified time zone
     */
    public static JulianChronology getInstance(DateTimeZone zone) {
        return getInstance(zone, 4);
    }

    
Gets an instance of the JulianChronology in the given time zone.

Params:
  • zone –  the time zone to get the chronology in, null is default
  • minDaysInFirstWeek –  minimum number of days in first week of the year; default is 4
Returns:a chronology in the specified time zone
/**
     * Gets an instance of the JulianChronology in the given time zone.
     * 
     * @param zone  the time zone to get the chronology in, null is default
     * @param minDaysInFirstWeek  minimum number of days in first week of the year; default is 4
     * @return a chronology in the specified time zone
     */
    public static JulianChronology getInstance(DateTimeZone zone, int minDaysInFirstWeek) {
        if (zone == null) {
            zone = DateTimeZone.getDefault();
        }
        JulianChronology chrono;
        JulianChronology[] chronos = cCache.get(zone);
        if (chronos == null) {
            chronos = new JulianChronology[7];
            JulianChronology[] oldChronos = cCache.putIfAbsent(zone, chronos);
            if (oldChronos != null) {
                chronos = oldChronos;
            }
        }
        try {
            chrono = chronos[minDaysInFirstWeek - 1];
        } catch (ArrayIndexOutOfBoundsException e) {
            throw new IllegalArgumentException
                ("Invalid min days in first week: " + minDaysInFirstWeek);
        }
        if (chrono == null) {
            synchronized (chronos) {
                chrono = chronos[minDaysInFirstWeek - 1];
                if (chrono == null) {
                    if (zone == DateTimeZone.UTC) {
                        chrono = new JulianChronology(null, null, minDaysInFirstWeek);
                    } else {
                        chrono = getInstance(DateTimeZone.UTC, minDaysInFirstWeek);
                        chrono = new JulianChronology
                            (ZonedChronology.getInstance(chrono, zone), null, minDaysInFirstWeek);
                    }
                    chronos[minDaysInFirstWeek - 1] = chrono;
                }
            }
        }
        return chrono;
    }

    // Constructors and instance variables
    //-----------------------------------------------------------------------

    
Restricted constructor

/**
     * Restricted constructor
     */
    JulianChronology(Chronology base, Object param, int minDaysInFirstWeek) {
        super(base, param, minDaysInFirstWeek);
    }

    
Serialization singleton

/**
     * Serialization singleton
     */
    private Object readResolve() {
        Chronology base = getBase();
        int minDays = getMinimumDaysInFirstWeek();
        minDays = (minDays == 0 ? 4 : minDays);  // handle rename of BaseGJChronology
        return base == null ?
                getInstance(DateTimeZone.UTC, minDays) :
                    getInstance(base.getZone(), minDays);
    }

    // Conversion
    //-----------------------------------------------------------------------
    
Gets the Chronology in the UTC time zone.

Returns:the chronology in UTC
/**
     * Gets the Chronology in the UTC time zone.
     * 
     * @return the chronology in UTC
     */
    public Chronology withUTC() {
        return INSTANCE_UTC;
    }

    
Gets the Chronology in a specific time zone.

Params:
  • zone –  the zone to get the chronology in, null is default
Returns:the chronology
/**
     * Gets the Chronology in a specific time zone.
     * 
     * @param zone  the zone to get the chronology in, null is default
     * @return the chronology
     */
    public Chronology withZone(DateTimeZone zone) {
        if (zone == null) {
            zone = DateTimeZone.getDefault();
        }
        if (zone == getZone()) {
            return this;
        }
        return getInstance(zone);
    }

    long getDateMidnightMillis(int year, int monthOfYear, int dayOfMonth)
        throws IllegalArgumentException
    {
        return super.getDateMidnightMillis(adjustYearForSet(year), monthOfYear, dayOfMonth);
    }

    boolean isLeapYear(int year) {
        return (year & 3) == 0;
    }

    long calculateFirstDayOfYearMillis(int year) {
        // Java epoch is 1970-01-01 Gregorian which is 1969-12-19 Julian.
        // Calculate relative to the nearest leap year and account for the
        // difference later.

        int relativeYear = year - 1968;
        int leapYears;
        if (relativeYear <= 0) {
            // Add 3 before shifting right since /4 and >>2 behave differently
            // on negative numbers.
            leapYears = (relativeYear + 3) >> 2;
        } else {
            leapYears = relativeYear >> 2;
            // For post 1968 an adjustment is needed as jan1st is before leap day
            if (!isLeapYear(year)) {
                leapYears++;
            }
        }
        
        long millis = (relativeYear * 365L + leapYears) * (long)DateTimeConstants.MILLIS_PER_DAY;

        // Adjust to account for difference between 1968-01-01 and 1969-12-19.

        return millis - (366L + 352) * DateTimeConstants.MILLIS_PER_DAY;
    }

    int getMinYear() {
        return MIN_YEAR;
    }

    int getMaxYear() {
        return MAX_YEAR;
    }

    long getAverageMillisPerYear() {
        return MILLIS_PER_YEAR;
    }

    long getAverageMillisPerYearDividedByTwo() {
        return MILLIS_PER_YEAR / 2;
    }

    long getAverageMillisPerMonth() {
        return MILLIS_PER_MONTH;
    }

    long getApproxMillisAtEpochDividedByTwo() {
        return (1969L * MILLIS_PER_YEAR + 352L * DateTimeConstants.MILLIS_PER_DAY) / 2;
    }

    protected void assemble(Fields fields) {
        if (getBase() == null) {
            super.assemble(fields);
            // Julian chronology has no year zero.
            fields.year = new SkipDateTimeField(this, fields.year);
            fields.weekyear = new SkipDateTimeField(this, fields.weekyear);
        }
    }

}