Used to grok the `now` parameter to createClock.
@param epoch {Date|number} the system time
function getEpoch(epoch) {
        if (!epoch) { return 0; }
        if (typeof epoch.getTime === "function") { return epoch.getTime(); }
        if (typeof epoch === "number") { return epoch; }
        throw new TypeError("now should be milliseconds since UNIX epoch");
    }