Vaadin Digital Clock
The client-side widget, after instantiation, maintains 2 GWT timer objects. One timer is scheduled to fetch the server time every 5 minutes from the web-app server. The other timer is scheduled to execute every second to keep the client-side clock ticking.
The information (or STATE) that is transferred between the client-side widget & the server-side component is a long variable called "time" which holds the current server time in milliseconds.
GWT RPC is used in the background to invoke the piece of code in the server-side component that actually calculates the current server time.