import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;
import java.util.TimeZone;

public class GmtDate {
	public static void main(String[] args){
		
		System.out.println(new Date());
		
		//本地日期格式.
    	String rfc1123_1 = new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss z",Locale.US).format(new Date());
    	System.out.println("rfc1123_1 = " + rfc1123_1);
    	
    	//错误
    	String rfc1123_2 = new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss 'GMT'",Locale.US).format(new Date());
    	System.out.println("rfc1123_2 = " + rfc1123_2);

    	//正确， 推荐使用.
    	SimpleDateFormat sdf3 = new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss z",Locale.US);
    	sdf3.setTimeZone(TimeZone.getTimeZone("GMT"));
    	String rfc1123_3 = sdf3.format(new Date());
    	System.out.println("rfc1123_3 = "+rfc1123_3);
    	
    	//正确.
    	SimpleDateFormat sdf4 = new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss 'GMT'",Locale.US);
    	sdf4.setTimeZone(TimeZone.getTimeZone("GMT"));
    	String rfc1123_4 = sdf4.format(new Date());
    	System.out.println("rfc1123_4 = " + rfc1123_4);
	}
}

输出的结果

Tue Oct 30 15:33:48 CST 2018
rfc1123_1 = Tue, 30 Oct 2018 15:33:48 CST
rfc1123_2 = Tue, 30 Oct 2018 15:33:48 GMT
rfc1123_3 = Tue, 30 Oct 2018 07:33:48 GMT
rfc1123_4 = Tue, 30 Oct 2018 07:33:48 GMT

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对应的命令

date=`env LANG="en_US.UTF-8" date -u "+%a, %d %b %Y %H:%M:%S GMT"`