code: def get_prep_value(self, value):
          
          log.debug("get_prep_value: {}, {}", value, type(value))
          if not value:
              return ''
              # For underlying (database) string types, e.g. VARCHAR, this
              # function must always return a string type.
              # https://docs.djangoproject.com/en/1.8/howto/custom-model-fields/
          # Convert to UTC
          return value.astimezone(timezone.utc)
docs: Convert Python value to database value for QUERYING.
          We query with UTC, so this function converts datetime values to UTC.
  
          Calls to this function are followed by calls to ``get_db_prep_value()``,
          which is for backend-specific conversions.