It depends on local timezone, you need Calendar
Timestamp t = ...
Calendar c = Calendar.getInstance();
c.setTimeInMillis(t.getTime());
c.set(Calendar.HOUR_OF_DAY, 0);
c.set(Calendar.MINUTE, 0);
c.set(Calendar.SECOND, 0);
c.set(Calendar.MILLISECOND, 0);
t = new Timestamp(c.getTimeInMillis());