code: def get_backup_file_time_tag(file_name, custom_prefix="backup"):
      
      name_string = file_name[len(custom_prefix):]
      time_tag = name_string.split(".", 1)[0]
      return datetime.strptime(time_tag, DATETIME_FORMAT)
docs: Returns a datetime object computed from a file name string, with
          formatting based on DATETIME_FORMAT.