AWS Athena and date_format

I have some issue while formatting a timestamp with Amazon Athena service.

select date_format(current_timestamp, 'y')

Returns just 'y' (the string).

The only way I found to format dates in Amazon Athena is trough CONCAT + YEAR + MONTH + DAY functions, like this:

select CONCAT(cast(year(current_timestamp) as varchar), '_', cast(day(current_timestamp) as varchar))