func: function getFirstDocumentIdBetweenDatesValidFrom(uint _unixTimeStarting, uint _unixTimeEnding) public view
      returns (uint firstID, uint lastId) 
      {
          firstID = 0;
          lastId = 0;
          
          for (uint i = 0; i < documentsCount; i++) {
              Document memory doc = documents[i];
              if (firstID==0) {
                  if (doc.validFrom>=_unixTimeStarting) {
                      firstID = i;
                  }
              } else {
                  if (doc.validFrom<=_unixTimeEnding) {
                      lastId = i;
                  }
              }
          }
          
          if ((firstID>0)&&(lastId==0)&&(_unixTimeStarting<_unixTimeEnding)) {
              lastId = documentsCount;
          }
      }