Java - How to get current date independent from system date? [duplicate]

Answer 1

Here's some code to fetch the date in HTTP format (usually UTC timezone) from a web server of your choice.

Of course, if you have no control over the physical hardware and OS, nothing can guarantee you will be able to talk to the actual web server you ask for... but anyway.

package some.package;

import java.io.IOException;
import java.net.URL;
import java.net.URLConnection;
import java.util.List;
import java.util.Map;

public class Test {

  private static String getServerHttpDate(String serverUrl) throws IOException {
    URL url = new URL(serverUrl);
    URLConnection connection = url.openConnection();

    Map<String, List<String>> httpHeaders = connection.getHeaderFields();

    for (Map.Entry<String, List<String>> entry : httpHeaders.entrySet()) {
      String headerName = entry.getKey();
      if (headerName != null && headerName.equalsIgnoreCase("date")) {
        return entry.getValue().get(0);
      }
    }
    return null;
  }

  public static void main(String[] args) throws IOException {
    String serverUrl = args.length > 0 ? args[0] : "https://google.com";
    System.out.println(getServerHttpDate(serverUrl));
  }

}

Answer 2

The programming level methods are developed to Get Date and Time from your system itself. You cannot modify them to get the dates except than the System specified.

For your additional requirement, if you wish to really have it, you would require a Synchronization between your Client Machine and Server.

Answer 3

If you're on the internet, you might be able to ask a known and trusted time source. If the person running your program wants to prevent your program from doing that (like if you've given them a time limited license and they don't want to pay for more time), they might spoof or block that connection.

On one project I was on, we placed a secure, trusted time source in the hardware that could not be tampered with. It was designed for encryption and licensing, and had a Java library to access it. Sorry, I can't remember the name of the device.

So the answer is maybe yes, maybe no.

Answer 4

In versions of Java prior to 1.1 it was standard to use the Date class:

Date now = new Date();     // Gets the current date and time
int year = now.getYear();  // Returns the # of years since 1900

However, in newer versions of Java, much of the Date class has been deprecated (specifically the getYear method). It is now more standard to use the Calendar class:

Calendar now = Calendar.getInstance();   // Gets the current date and time
int year = now.get(Calendar.YEAR);       // The current year

Answer 5

Have your system Internet access? If so, you can use synchronization with precise time services (for example: http://tldp.org/HOWTO/TimePrecision-HOWTO/ntp.html) and grant that you want.

Answer 6

I don't understand completely your question but I can answer your title:

GregorianCalendar gc = new GregorianCalendar(System.getCurrentTimeMillis());
int year = gc.get(Calendar.YEAR);