You can get the source code for the ClockApplet from the SourceForge subversion repository. Instructions can be found here.
You can use Maven 2 to build the ClockApplet from source. Once you have Maven installed. Change to the clock applet directory and:
mvn compile
In order to access an outside NTP server, the applet must be signed. The binary distribution is signed with a bogus self-signed certificate, and causes a dialog box to appear for user approval before the applet can be run.
These instructions will allow you to create your own self-signed certificate. Most production deployments will require the applet to be signed by a well-known certificate authority. Instructions for that process can be found elsewhere on the web and will not be reproduced here.
Use the keytool provided with your Java distribution to create a public/private key pair (and possibly a keystore at the same time):
keytool -genkey -alias clockapplet
Your key pair need not be called clockapplet, but it seems like a convenient name for now. You will be prompted for information about you and your organization. Enter as much as you like. You will also be prompted for a key password (and possibly a keystore password).
Then all that's left is to run the jarsigner tool, which can be done with the exeuction attached to the maven 'package' lifecycle element. Replace 'myStorePass' and 'myKeyPass' with the store password and key password from the previous step.
mvn -Dsign.cert.alias=clockapplet \
-Dsign.store.pass=myStorePass -Dsign.key.pass=myKeyPass \
package
The jar now found in the 'target' subdirectory will be signed.