gistfile1.js

// ==UserScript==
// @name           twitter timestamps
// @namespace      rjp
// @include        http://*twitter.com/*
// ==/UserScript==

var allTimestamps, thisTimestamp;
allTimestamps = document.evaluate("//span[contains(@class,'timestamp')]",
  document,
  null,
  XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE,
  null);

for (var i=0;i<allTimestamps.snapshotLength;i++) {
 var thisTimestamp = allTimestamps.snapshotItem(i);
 var nicetime = thisTimestamp.getAttribute('data');
 var tval = eval(nicetime);
 var mytimezone = new Date(tval);
	thisTimestamp.innerHTML = mytimezone;
}