Java: How to add seconds to Timestamp?

I fail to add seconds to Java Timestamp.

I have this, but, it gives me the same date:

int sec = 600;

java.sql.Timestamp ts_from_ws = new java.sql.Timestamp(retry_date);
Calendar cal = Calendar.getInstance();
cal.setTimeInMillis(ts_from_ws.getTime());
cal.add(Calendar.SECOND,sec);
java.sql.Timestamp ts_new_date_ws = new java.sql.Timestamp(cal.getTime().getTime());