NTPTimeUtil.java

/*
*
* 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.
*/ 
import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.TimeZone;

/**
 * time converter between the NTP time and the java date time class Calendar
 * 
 * @author Steven Ou
 *
 */
public class NTPTimeUtil {

	private static final long NTP_GAP_SECONDS = 2208988800L;
	
	/**
	 * convert the NTP time to Java Date time
	 * @param ntpSeconds NTP time in seconds
	 * @return the Calendar
	 */
	public Calendar toDateTime( long ntpSeconds ) {
		Calendar cal = Calendar.getInstance( TimeZone.getTimeZone("UTC") );
		cal.setTimeInMillis( ( ntpSeconds - NTP_GAP_SECONDS ) * 1000 );
		return cal;
	}
	
	/**
	 * convert the date time <code>cal</code> to UTC time zone
	 * @param cal the date time
	 * @return the date time in UTC
	 */
	public static Calendar toUTC( Calendar cal ) {
		TimeZone tz = cal.getTimeZone();
		int offset = tz.getOffset( cal.getTimeInMillis() );
		
		if( offset == 0 ) {
			return cal;
		} else {
			GregorianCalendar gcal = new GregorianCalendar( TimeZone.getTimeZone("GMT") );
			gcal.setTimeInMillis( cal.getTimeInMillis() + offset );
			return gcal;
		}
	}
	
	/**
	 * convert datetime to NTP time in seconds
	 * @param cal the date time
	 * @return the NTP time in seconds
	 */
	public long toNTPSeconds( Calendar cal ) {
		return toUTC( cal ).getTimeInMillis()/1000 + NTP_GAP_SECONDS;
	}
}