Would it be better to make this into a helper function (getFeedPageHelper), and have 2 versions of getFeedPage:

   * getFeedPage(GetFeedRequest getFeedRequest, String marker, int pageSize) 
   * getFeedPage(GetFeedRequest getFeedRequest, Date markerTimestamp, int pageSize) 

Each of these methods would call getFeedPageHelper(...) and generate any missing arguments.

This would make using getFeedPage() less error prone & improve readability.