code: function convertDates (obj, convert) {
    obj = clone(obj);
    for (var key in obj) {
      var val = obj[key];
      if (is.date(val)) obj[key] = convert(val);
      if (is.object(val)) obj[key] = convertDates(val, convert);
    }
    return obj;
  }
docs: Recursively convert an `obj`'s dates to new values.
  
  @param {Object} obj
  @param {Function} convert
  @return {Object}