Since the 2.1 implementation of the filter remembers the time at which the data was cached, then now we may send to the client/browser/proxy some headers as to when the data will expire.
In order to do this I do the following:

CacheFilter.java: line 75:
((HttpServletResponse)response).setDateHeader(
     "Expires",
     respContent.getLastModified()+time*1000
);

also line 98:
((HttpServletResponse)response).setDateHeader(
      "Expires",
      System.currentTimeMillis()+time*1000
);