This example is a Sum for Date time and Time Zone(String Values)
String DateVal = "2015-03-26 12:00:00";
String TimeVal = "02:00:00";
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
SimpleDateFormat sdf2 = new SimpleDateFormat("HH:mm:ss");
Date reslt = sdf.parse( DateVal );
Date timeZ = sdf2.parse( TimeVal );
//Increase Date Time
reslt.setHours( reslt.getHours() + timeZ.getHours());
reslt.setMinutes( reslt.getMinutes() + timeZ.getMinutes());
reslt.setSeconds( reslt.getSeconds() + timeZ.getSeconds());
System.printLn.out( sdf.format(reslt) );//Result(+2 Hours): 2015-03-26 14:00:00
Thanks :)