http://www.joda.org/joda-time/: Date and time library to replace JDK date handling (Joda.org) 
/*
 *  Copyright 2001-2013 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 org.joda.time.DateTimeConstants;
import org.joda.time.DateTimeFieldType;
import org.joda.time.DateTimeUtils;
import org.joda.time.DurationField;
import org.joda.time.ReadablePartial;
import org.joda.time.field.FieldUtils;
import org.joda.time.field.ImpreciseDateTimeField;


Provides time calculations for the month of the year component of time.

Author:Guy Allard, Stephen Colebourne, Brian S O'Neill
Since:1.2, refactored from GJMonthOfYearDateTimeField
/**
 * Provides time calculations for the month of the year component of time.
 *
 * @author Guy Allard
 * @author Stephen Colebourne
 * @author Brian S O'Neill
 * @since 1.2, refactored from GJMonthOfYearDateTimeField
 */
class BasicMonthOfYearDateTimeField extends ImpreciseDateTimeField {

    
Serialization version 
/** Serialization version */
    @SuppressWarnings("unused")
    private static final long serialVersionUID = -8258715387168736L;

    private static final int MIN = DateTimeConstants.JANUARY;

    private final BasicChronology iChronology;
    private final int iMax;
    private final int iLeapMonth;

    
Restricted constructor.

Params:
  • leapMonth – the month of year that leaps
/**
     * Restricted constructor.
     * 
     * @param leapMonth the month of year that leaps
     */
    BasicMonthOfYearDateTimeField(BasicChronology chronology, int leapMonth) {
        super(DateTimeFieldType.monthOfYear(), chronology.getAverageMillisPerMonth());
        iChronology = chronology;
        iMax = iChronology.getMaxMonth();
        iLeapMonth = leapMonth;
    }

    //-----------------------------------------------------------------------
    public boolean isLenient() {
        return false;
    }

    //-----------------------------------------------------------------------
    
Get the Month component of the specified time instant.

Params:
  • instant –  the time instant in millis to query.
See Also:
Returns:the month extracted from the input.
/**
     * Get the Month component of the specified time instant.
     *
     * @see org.joda.time.DateTimeField#get(long)
     * @see org.joda.time.ReadableDateTime#getMonthOfYear()
     * @param instant  the time instant in millis to query.
     * @return the month extracted from the input.
     */
    public int get(long instant) {
        return iChronology.getMonthOfYear(instant);
    }

    //-----------------------------------------------------------------------
    
Add the specified month to the specified time instant.
The amount added may be negative.

If the new month has less total days than the specified
day of the month, this value is coerced to the nearest
sane value. e.g.

07-31 - (1 month) = 06-30

03-31 - (1 month) = 02-28 or 02-29 depending


Params:
  • instant –  the time instant in millis to update.
  • months –  the months to add (can be negative).
See Also:
Returns:the updated time instant.
/**
     * Add the specified month to the specified time instant.
     * The amount added may be negative.<p>
     * If the new month has less total days than the specified
     * day of the month, this value is coerced to the nearest
     * sane value. e.g.<p>
     * 07-31 - (1 month) = 06-30<p>
     * 03-31 - (1 month) = 02-28 or 02-29 depending<p>
     * 
     * @see org.joda.time.DateTimeField#add
     * @see org.joda.time.ReadWritableDateTime#addMonths(int)
     * @param instant  the time instant in millis to update.
     * @param months  the months to add (can be negative).
     * @return the updated time instant.
     */
    public long add(long instant, int months) {
        if (months == 0) {
            return instant; // the easy case
        }
        //
        // Save time part first.
        //
        long timePart = iChronology.getMillisOfDay(instant);
        //
        //
        // Get this year and month.
        //
        int thisYear = iChronology.getYear(instant);
        int thisMonth = iChronology.getMonthOfYear(instant, thisYear);
        // ----------------------------------------------------------
        //
        // Do not refactor without careful consideration.
        // Order of calculation is important.
        //
        int yearToUse = thisYear;;
        // Initially, monthToUse is zero-based
        int monthToUse = thisMonth - 1 + months;
        if (thisMonth > 0 && monthToUse < 0) {
            if (Math.signum(months + iMax) == Math.signum(months)) {
                yearToUse--;
                months += iMax;
            } else {
                yearToUse++;
                months -= iMax;
            }
            monthToUse = thisMonth - 1 + months;
        }
        if (monthToUse >= 0) {
            yearToUse = yearToUse + (monthToUse / iMax);
            monthToUse = (monthToUse % iMax) + 1;
        } else {
            yearToUse = yearToUse + (monthToUse / iMax) - 1;
            monthToUse = Math.abs(monthToUse);
            int remMonthToUse = monthToUse % iMax;
            // Take care of the boundary condition
            if (remMonthToUse == 0) {
                remMonthToUse = iMax;
            }
            monthToUse = iMax - remMonthToUse + 1;
            // Take care of the boundary condition
            if (monthToUse == 1) {
                yearToUse += 1;
            }
        }
        // End of do not refactor.
        // ----------------------------------------------------------

        //
        // Quietly force DOM to nearest sane value.
        //
        int dayToUse = iChronology.getDayOfMonth(instant, thisYear, thisMonth);
        int maxDay = iChronology.getDaysInYearMonth(yearToUse, monthToUse);
        if (dayToUse > maxDay) {
            dayToUse = maxDay;
        }
        //
        // get proper date part, and return result
        //
        long datePart =
            iChronology.getYearMonthDayMillis(yearToUse, monthToUse, dayToUse);
        return datePart + timePart;
    }

    //-----------------------------------------------------------------------
    public long add(long instant, long months) {
        int i_months = (int)months;
        if (i_months == months) {
            return add(instant, i_months);
        }

        // Copied from add(long, int) and modified slightly:

        long timePart = iChronology.getMillisOfDay(instant);

        int thisYear = iChronology.getYear(instant);
        int thisMonth = iChronology.getMonthOfYear(instant, thisYear);

        long yearToUse;
        long monthToUse = thisMonth - 1 + months;
        if (monthToUse >= 0) {
            yearToUse = thisYear + (monthToUse / iMax);
            monthToUse = (monthToUse % iMax) + 1;
        } else {
            yearToUse = thisYear + (monthToUse / iMax) - 1;
            monthToUse = Math.abs(monthToUse);
            int remMonthToUse = (int)(monthToUse % iMax);
            if (remMonthToUse == 0) {
                remMonthToUse = iMax;
            }
            monthToUse = iMax - remMonthToUse + 1;
            if (monthToUse == 1) {
                yearToUse += 1;
            }
        }

        if (yearToUse < iChronology.getMinYear() ||
            yearToUse > iChronology.getMaxYear()) {

            throw new IllegalArgumentException
                ("Magnitude of add amount is too large: " + months);
        }

        int i_yearToUse = (int)yearToUse;
        int i_monthToUse = (int)monthToUse;

        int dayToUse = iChronology.getDayOfMonth(instant, thisYear, thisMonth);
        int maxDay = iChronology.getDaysInYearMonth(i_yearToUse, i_monthToUse);
        if (dayToUse > maxDay) {
            dayToUse = maxDay;
        }

        long datePart =
            iChronology.getYearMonthDayMillis(i_yearToUse, i_monthToUse, dayToUse);
        return datePart + timePart;
    }

    //-----------------------------------------------------------------------
    public int[] add(ReadablePartial partial, int fieldIndex, int[] values, int valueToAdd) {
        // overridden as superclass algorithm can't handle
        // 2004-02-29 + 48 months -> 2008-02-29 type dates
        if (valueToAdd == 0) {
            return values;
        }
        if (partial.size() > 0 && partial.getFieldType(0).equals(DateTimeFieldType.monthOfYear()) && fieldIndex == 0) {
            // month is largest field and being added to, such as month-day
            int curMonth0 = values[0] - 1;
            int newMonth = ((curMonth0 + (valueToAdd % 12) + 12) % 12) + 1;
            return set(partial, 0, values, newMonth);
        }
        if (DateTimeUtils.isContiguous(partial)) {
            long instant = 0L;
            for (int i = 0, isize = partial.size(); i < isize; i++) {
                instant = partial.getFieldType(i).getField(iChronology).set(instant, values[i]);
            }
            instant = add(instant, valueToAdd);
            return iChronology.get(partial, instant);
        } else {
            return super.add(partial, fieldIndex, values, valueToAdd);
        }
    }

    //-----------------------------------------------------------------------
    
Add to the Month component of the specified time instant
wrapping around within that component if necessary.

Params:
  • instant –  the time instant in millis to update.
  • months –  the months to add (can be negative).
See Also:
  • addWrapField.addWrapField
Returns:the updated time instant.
/**
     * Add to the Month component of the specified time instant
     * wrapping around within that component if necessary.
     * 
     * @see org.joda.time.DateTimeField#addWrapField
     * @param instant  the time instant in millis to update.
     * @param months  the months to add (can be negative).
     * @return the updated time instant.
     */
    public long addWrapField(long instant, int months) {
        return set(instant, FieldUtils.getWrappedValue(get(instant), months, MIN, iMax));
    }

    //-----------------------------------------------------------------------
    public long getDifferenceAsLong(long minuendInstant, long subtrahendInstant) {
        if (minuendInstant < subtrahendInstant) {
            return -getDifference(subtrahendInstant, minuendInstant);
        }

        int minuendYear = iChronology.getYear(minuendInstant);
        int minuendMonth = iChronology.getMonthOfYear(minuendInstant, minuendYear);
        int subtrahendYear = iChronology.getYear(subtrahendInstant);
        int subtrahendMonth = iChronology.getMonthOfYear(subtrahendInstant, subtrahendYear);

        long difference = (minuendYear - subtrahendYear) * ((long) iMax) + minuendMonth - subtrahendMonth;

        // Before adjusting for remainder, account for special case of add
        // where the day-of-month is forced to the nearest sane value.
        int minuendDom = iChronology.getDayOfMonth
            (minuendInstant, minuendYear, minuendMonth);
        if (minuendDom == iChronology.getDaysInYearMonth(minuendYear, minuendMonth)) {
            // Last day of the minuend month...
            int subtrahendDom = iChronology.getDayOfMonth
                (subtrahendInstant, subtrahendYear, subtrahendMonth);
            if (subtrahendDom > minuendDom) {
                // ...and day of subtrahend month is larger.
                // Note: This works fine, but it ideally shouldn't invoke other
                // fields from within a field.
                subtrahendInstant = iChronology.dayOfMonth().set(subtrahendInstant, minuendDom);
            }
        }

        // Inlined remainder method to avoid duplicate calls.
        long minuendRem = minuendInstant
            - iChronology.getYearMonthMillis(minuendYear, minuendMonth);
        long subtrahendRem = subtrahendInstant
            - iChronology.getYearMonthMillis(subtrahendYear, subtrahendMonth);

        if (minuendRem < subtrahendRem) {
            difference--;
        }

        return difference;
    }

    //-----------------------------------------------------------------------
    
Set the Month component of the specified time instant.

If the new month has less total days than the specified
day of the month, this value is coerced to the nearest
sane value. e.g.

07-31 to month 6 = 06-30

03-31 to month 2 = 02-28 or 02-29 depending


Params:
  • instant –  the time instant in millis to update.
  • month –  the month (1,12) to update the time to.
Throws:
Returns:the updated time instant.
/**
     * Set the Month component of the specified time instant.<p>
     * If the new month has less total days than the specified
     * day of the month, this value is coerced to the nearest
     * sane value. e.g.<p>
     * 07-31 to month 6 = 06-30<p>
     * 03-31 to month 2 = 02-28 or 02-29 depending<p>
     * 
     * @param instant  the time instant in millis to update.
     * @param month  the month (1,12) to update the time to.
     * @return the updated time instant.
     * @throws IllegalArgumentException  if month is invalid
     */
    public long set(long instant, int month) {
        FieldUtils.verifyValueBounds(this, month, MIN, iMax);
        //
        int thisYear = iChronology.getYear(instant);
        //
        int thisDom = iChronology.getDayOfMonth(instant, thisYear);
        int maxDom = iChronology.getDaysInYearMonth(thisYear, month);
        if (thisDom > maxDom) {
            // Quietly force DOM to nearest sane value.
            thisDom = maxDom;
        }
        // Return newly calculated millis value
        return iChronology.getYearMonthDayMillis(thisYear, month, thisDom) +
            iChronology.getMillisOfDay(instant);
    }

    //-----------------------------------------------------------------------
    public DurationField getRangeDurationField() {
        return iChronology.years();
    }

    //-----------------------------------------------------------------------
    public boolean isLeap(long instant) {
        int thisYear = iChronology.getYear(instant);
        if (iChronology.isLeapYear(thisYear)) {
            return (iChronology.getMonthOfYear(instant, thisYear) == iLeapMonth);
        }
        return false;
    }

    //-----------------------------------------------------------------------
    public int getLeapAmount(long instant) {
        return isLeap(instant) ? 1 : 0;
    }

    //-----------------------------------------------------------------------
    public DurationField getLeapDurationField() {
        return iChronology.days();
    }

    //-----------------------------------------------------------------------
    public int getMinimumValue() {
        return MIN;
    }

    //-----------------------------------------------------------------------
    public int getMaximumValue() {
        return iMax;
    }

    //-----------------------------------------------------------------------
    public long roundFloor(long instant) {
        int year = iChronology.getYear(instant);
        int month = iChronology.getMonthOfYear(instant, year);
        return iChronology.getYearMonthMillis(year, month);
    }

    //-----------------------------------------------------------------------
    public long remainder(long instant) {
        return instant - roundFloor(instant);
    }

    //-----------------------------------------------------------------------
    
Serialization singleton

/**
     * Serialization singleton
     */
    private Object readResolve() {
        return iChronology.monthOfYear();
    }
}