Why System.currentTimeMillis() generate incorrect long value?

Why System.currentTimeMillis() generate incorrect long value?

It was discussed too many times, I know, but I can't get why the
milliseconds generated by mine:
System.currentTimeMillis();
Or by:
Calendar.getInstance(TimeZone.getTimeZone("UTC")).getTimeInMillis()
Are not equal to what I see on http://www.epochconverter.com/?
What I need is to merely generate a String of concrete format, but I've
found out the milliseconds aren't right.
Just in case here is how I do it:
private static final String DATE_PATTERN = "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'";
public static String getCurrentTimestamp() {
long time = System.currentTimeMillis();
SimpleDateFormat sdf = new SimpleDateFormat(DATE_PATTERN);
String lastModifiedTime = sdf.format(time);
Logger.logVerbose(TAG, "Generated timestamp is " + lastModifiedTime);
return lastModifiedTime;
}
What I finally get is just a local time, but I need the only time which is
pure UTC without conjunction with my timezone.
I've even checked it with SQLite (using the SELECT
strftime('%s',timestring);) and got the correct milliseconds. Why then I
got it incorrectly generated by those two statements I posted above?
Thanks a lot in advance.