Ah ok, thats where you are coming from.
JavaScript/ECMAScript defines that JavaScript Date supports exactly 100000000 days before and after unix epoch. This gives us a maximum year of 275760 that JavaScript Date can currently represent.
Java itself uses a long to represent millis since epoch in java.util.Date. Because long is 64 bit the maximum year that java.util.Date can represent is 292278994.
JavaScript: 275760
Java: 292278994
Since GWT emulates java.util.Date using native JavaScript Date the max year in GWT code is the one of JavaScript.
In any case year 2038 shouldn't be a problem, thats why I asked. You can easily verify it in browser console using
var millis = Date.parse("2040-06-01");
var date = new Date(millis);
console.log(date);
-- J.