Please check access policy for each server before use, http://www.eecis.udel.edu/~mills/ntp/servers.html, then add:
grant codeBase "http://home.elka.pw.edu.pl/~ptrojane/*" {
  permission java.net.SocketPermission "*:123", "connect,resolve,accept";
to your Java user policy file. This makes possible to send/receive UDP packets to NTP servers, which listens on port 123 by applets downloaded from this site.
The user policy file is by default located at

user.home/.java.policy  (Solaris)
user.home\.java.policy  (Win32)

Note: user.home refers to the value of the system property named
"user.home", which specifies the user's home directory. On Win32 systems,
given user name uName, the "user.home" property value defaults to

C:\Documents and Settings\uName on Windows 2000
C:\Winnt\Profiles\uName on multi-user Windows NT systems
C:\Windows\Profiles\uName on multi-user Windows 95 systems
C:\Windows on single-user Windows 95 systems