import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.text.SimpleDateFormat;
import java.util.Calendar;

public class Test {
    public static void main(String[] args) throws Exception {
        String startstr = Test.readSystemStartTime();
        String startdate = startstr.substring(7,startstr.length());

        Calendar c = Calendar.getInstance();
        c.setTime(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse(startdate));
        System.out.println("开前到现在的毫秒数：" + (System.currentTimeMillis() - c.getTimeInMillis()));
    }

    public static String readSystemStartTime() throws IOException, InterruptedException {
        Process process = Runtime.getRuntime().exec("cmd /c net statistics workstation");
        String startUpTime = "";
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(process.getInputStream(), "GBK"));
        int i = 0;
        while ((startUpTime = bufferedReader.readLine()) != null) {
            if (i == 3) {
                break;
            }
            i++;
        }
        process.waitFor();
        return startUpTime;
    }
}

------解决方案--------------------
String startdate = startstr.substring(7,startstr.length());

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不同的系统可能显示的不一样