The "works in" browser icons gives some idea of the date on that. IE, Phoenix, Netscape 4, and Opera. Probably some of the oldest still-in-use javascript in the world :P

Ran it through prettier.io:

    javascript: (function() {
        var e, s;
        IB = 1;
        function isDigit(c) {
            return "0" <= c && c <= "9";
        }
        L = location.href;
        LL = L.length;
        for (e = LL - 1; e >= 0; --e)
            if (isDigit(L.charAt(e))) {
            for (s = e - 1; s >= 0; --s) if (!isDigit(L.charAt(s))) break;
            break;
            }
        ++s;
        if (e < 0) return;
        oldNum = L.substring(s, e + 1);
        newNum = "" + (parseInt(oldNum, 10) + IB);
        while (newNum.length < oldNum.length) newNum = "0" + newNum;
        location.href = L.substring(0, s) + newNum + L.slice(e + 1);
    })();