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");
}
Used to grok the `now` parameter to createClock.
@param epoch {Date|number} the system time