You haven't set the timezone only added a Z
to the end of the date/time, so it will look like a GMT date/time but this doesn't change the value.
Set the timezone to GMT and it will be correct.
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'");
sdf.setTimeZone(TimeZone.getTimeZone("GMT"));