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

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

    raise NotImplementedError('subclasses    of    BaseDatabaseOperations    may    require    a    datetime_trunc_sql()    method')
