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JACOB is a JAVA-COM Bridge that allows you to call COM Automation components from Java. It uses JNI to make native calls to the COM libraries. JACOB runs on x86 and x64 environments supporting 32 bit and 64 bit JVMs.

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/*
 * Copyright (c) 1999-2004 Sourceforge JACOB Project.
 * All rights reserved. Originator: Dan Adler (http://danadler.com).
 * Get more information about JACOB at http://sourceforge.net/projects/jacob-project
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
package com.jacob.com;

import java.util.Calendar;
import java.util.Date;

/**
 * java / windows date conversion utilities
 * 
 * @author joe
 * 
 */
public class DateUtilities {

	/**
	 * converts a windows time to a Java Date Object
	 * 
	 * @param comTime
	 * @return Date object representing the windows time as specified in comTime
	 */
	static public Date convertWindowsTimeToDate(double comTime) {
		return new Date(convertWindowsTimeToMilliseconds(comTime));
	}

	/**
	 * Convert a COM time from functions Date(), Time(), Now() to a Java time
	 * (milliseconds). Visual Basic time values are based to 30.12.1899, Java
	 * time values are based to 1.1.1970 (= 0 milliseconds). The difference is
	 * added to the Visual Basic value to get the corresponding Java value. The
	 * Visual Basic double value reads: .<1
	 * day percentage fraction>, e.g. "38100.6453" means: 38100 days since
	 * 30.12.1899 plus (24 hours * 0.6453). Example usage:
	 * Date javaDate = new Date(toMilliseconds (vbDate));.
	 * 
	 * @param comTime
	 *            COM time.
	 * @return Java time.
	 */
	static public long convertWindowsTimeToMilliseconds(double comTime) {
		long result = 0;

		// code from jacobgen:
		comTime = comTime - 25569D;
		Calendar cal = Calendar.getInstance();
		result = Math.round(86400000L * comTime)
				- cal.get(Calendar.ZONE_OFFSET);
		cal.setTime(new Date(result));
		result -= cal.get(Calendar.DST_OFFSET);

		return result;
	}// convertWindowsTimeToMilliseconds()

	/**
	 * converts a java date to a windows time object (is this timezone safe?)
	 * 
	 * @param javaDate
	 *            the java date to be converted to windows time
	 * @return the double representing the date in a form windows understands
	 */
	static public double convertDateToWindowsTime(Date javaDate) {
		if (javaDate == null) {
			throw new IllegalArgumentException(
					"cannot convert null to windows time");
		}
		return convertMillisecondsToWindowsTime(javaDate.getTime());
	}

	/**
	 * Convert a Java time to a COM time.
	 * 
	 * @param milliseconds
	 *            Java time.
	 * @return COM time.
	 */
	static public double convertMillisecondsToWindowsTime(long milliseconds) {
		double result = 0.0;

		// code from jacobgen:
		Calendar cal = Calendar.getInstance();
		cal.setTimeInMillis(milliseconds);
		milliseconds += (cal.get(Calendar.ZONE_OFFSET) + cal
				.get(Calendar.DST_OFFSET)); // add GMT offset
		result = (milliseconds / 86400000D) + 25569D;

		return result;
	}// convertMillisecondsToWindowsTime()
}