Getting Date in HTTP format in Java

In case someone else will try to find the answer here (like I did) here's what will do the trick:

String getServerTime() {
    Calendar calendar = Calendar.getInstance();
    SimpleDateFormat dateFormat = new SimpleDateFormat(
        "EEE, dd MMM yyyy HH:mm:ss z", Locale.US);
    dateFormat.setTimeZone(TimeZone.getTimeZone("GMT"));
    return dateFormat.format(calendar.getTime());
}

in order to set the server to speak English and give time in GMT timezone.

java.time

EDIT:

DateTimeFormatter.ofPattern("EEE, dd MMM yyyy HH:mm:ss z", Locale.ENGLISH).withZone(ZoneId.of("GMT"))

is the way to do it with pure java.time. HTTP 1.1 is to not a 100% match with RFC 1123, so using the java.time.format.DateTimeFormatter.RFC_1123_DATE_TIME formatter will fail for day-of-month less than 10. (thanks to @PavanKamar and @ankon for pointing that out)

Note: to be backwards compliant, you would need to also support the other two formats specified by RFC 2616