gistfile1.js

// relies on Date.now() which has been supported everywhere modern for years.
// as Safari 6 doesn't have support for NavigationTiming, we use a Date.now() timestamp for relative values
(function(window) {
    // prepare base perf object
    if ("performance" in window) {} else {
        window.performance = {};
    }
    if ("now" in window.performance) {} else {
        var nowOffset = Date.now();
        if ("timing" in window.performance && "navigationStart" in window.performance.timing) {
            nowOffset = window.performance.timing.navigationStart;
        }
        window.performance.now = function() {
            return Date.now() - nowOffset;
        };
    }
})(this);