title: System Time change detection on linux
body: <p>I was reading about the licensing of software and one question that came to my mind is that "how software detect the change in system time and block themselves if someone changes system time?". Since there is no other reference available(provided we don't have internet connection, otherwise we can use time servers as reference), how such software manages to do it? Are there standard algorithms or any standard libraries(C/C++) available for implementing the same. Please suggest.</p>