'Given a lookup_type of \'year\', \'month\', \'day\', \'hour\', \'minute\' or
\'second\', returns the SQL that truncates the given datetime field
field_name to a datetime object with only the given specificity, and
a tuple of parameters.'

def datetime_trunc_sql(self, lookup_type, field_name, tzname):

    raise NotImplementedError()
