code: def _AnalyzeKeywords(self, keywords):
      
      start_time = rdfvalue.RDFDatetime.Now() - rdfvalue.Duration("180d")
      filtered_keywords = []
  
      for k in keywords:
        if k.startswith(self.START_TIME_PREFIX):
          try:
            start_time = rdfvalue.RDFDatetime.FromHumanReadable(
                k[self.START_TIME_PREFIX_LEN:])
          except ValueError:
            pass
        else:
          filtered_keywords.append(k)
  
      if not filtered_keywords:
        filtered_keywords.append(".")
  
      return start_time, filtered_keywords
docs: Extracts a start time from a list of keywords if present.