code: public String getLastUpdatedString() {
          long newestTs = 0;
          for (UpdateSite s : sites) {
              if (s.getDataTimestamp()>newestTs) {
                  newestTs = s.getDataTimestamp();
              }
          }
          if (newestTs == 0) {
              return Messages.UpdateCenter_n_a();
          }
          return Util.getPastTimeString(System.currentTimeMillis()-newestTs);
      }
docs: Gets the string representing how long ago the data was obtained.
  Will be the newest of all {@link UpdateSite}s.