■ サンプル

import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;
import java.text.DateFormat;
import java.text.SimpleDateFormat;

import com.ibm.icu.util.Calendar;

public class Main {

  public static void main(String[] args) {
    Calendar calendar = getDatetimeFromResponseHeader("https://blogs.yahoo.co.jp/dk521123/37095462.html", 10_000);
    System.out.println(toString(calendar));
  }

  public static Calendar getDatetimeFromResponseHeader(String url, int timeout) {
    try {
      URL targetUrl = new URL(url);
      HttpURLConnection connection = (HttpURLConnection) targetUrl.openConnection();
      connection.setConnectTimeout(timeout);
      connection.setReadTimeout(timeout);
      connection.setRequestMethod("HEAD");
      Calendar returnValue = Calendar.getInstance();
      returnValue.setTimeInMillis(connection.getDate());
      return returnValue;
    } catch (IOException exception) {
      return null;
    }
  }
  
  public static String toString(Calendar calendar){
    DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss SSS");
    return dateFormat.format(calendar.getTime());
  }
}

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