docs: def process_pub_date(year, mon, day):
      """Create pub_date from what Pubmed provides in Journal PubDate entry
      """
  
      pub_date = None
      if year and re.match("[a-zA-Z]+", mon):
          pub_date = datetime.datetime.strptime(
              f"{year}-{mon}-{day}", "%Y-%b-%d"
          ).strftime("%Y-%m-%d")
      elif year:
          pub_date = f"{year}-{mon}-{day}"
  
      return pub_date