package org.apache.poi.ss.formula.functions;

import org.apache.poi.ss.formula.OperationEvaluationContext;

import org.apache.poi.ss.formula.eval.*;

import org.apache.poi.ss.usermodel.DateUtil;

import java.util.Calendar;

import java.util.Date;

public class EDate implements FreeRefFunction {

    @Override

    public ValueEval evaluate(ValueEval[] args, OperationEvaluationContext ec) {

        if (args.length != 2) {

            return ErrorEval.VALUE_INVALID;

        }

        try {

            double startDateAsNumber = getValue(args[0]);

            NumberEval offsetInYearsValue = (NumberEval) args[1];

            int offsetInMonthAsNumber = (int) offsetInYearsValue.getNumberValue();

            Date startDate = DateUtil.getJavaDate(startDateAsNumber);

            Calendar calendar = Calendar.getInstance();

            calendar.setTime(startDate);

            calendar.add(Calendar.MONTH, offsetInMonthAsNumber);

            return new NumberEval(DateUtil.getExcelDate(calendar.getTime()));

        } catch (EvaluationException e) {

            return e.getErrorEval();

        }

    }

    private double getValue(ValueEval arg) throws EvaluationException {

        if (arg instanceof NumberEval) {

            return ((NumberEval) arg).getNumberValue();

        }

        if (arg instanceof RefEval) {

            ValueEval innerValueEval = ((RefEval) arg).getInnerValueEval();

            return ((NumberEval) innerValueEval).getNumberValue();

        }

        throw new EvaluationException(ErrorEval.REF_INVALID);

    }

}

package org.apache.poi.ss.formula.functions;

import junit.framework.TestCase;

import org.apache.poi.ss.formula.eval.ErrorEval;

import org.apache.poi.ss.formula.eval.NumberEval;

import org.apache.poi.ss.formula.eval.ValueEval;

import org.apache.poi.ss.usermodel.DateUtil;

import org.apache.poi.ss.usermodel.ErrorConstants;

import java.util.Calendar;

import java.util.Date;

public class TestEDate extends TestCase{

    public void testEDateProperValues() {

        EDate eDate = new EDate();

        NumberEval result = (NumberEval) eDate.evaluate(new ValueEval[]{new NumberEval(1000), new NumberEval(0)}, null);

        assertEquals(1000d, result.getNumberValue());

    }

    public void testEDateInvalidValues() {

        EDate eDate = new EDate();

        ErrorEval result = (ErrorEval) eDate.evaluate(new ValueEval[]{new NumberEval(1000)}, null);

        assertEquals(ErrorConstants.ERROR_VALUE, result.getErrorCode());

    }

    public void testEDateIncrease() {

        EDate eDate = new EDate();

        Date startDate = new Date();

        int offset = 2;

        NumberEval result = (NumberEval) eDate.evaluate(new ValueEval[]{new NumberEval(DateUtil.getExcelDate(startDate)), new NumberEval(offset)}, null);

        Date resultDate = DateUtil.getJavaDate(result.getNumberValue());

        Calendar instance = Calendar.getInstance();

        instance.setTime(startDate);

        instance.add(Calendar.MONTH, offset);

        assertEquals(resultDate, instance.getTime());

    }

    public void testEDateDecrease() {

        EDate eDate = new EDate();

        Date startDate = new Date();

        int offset = -2;

        NumberEval result = (NumberEval) eDate.evaluate(new ValueEval[]{new NumberEval(DateUtil.getExcelDate(startDate)), new NumberEval(offset)}, null);

        Date resultDate = DateUtil.getJavaDate(result.getNumberValue());

        Calendar instance = Calendar.getInstance();

        instance.setTime(startDate);

        instance.add(Calendar.MONTH, offset);

        assertEquals(resultDate, instance.getTime());

    }

}