Create date custom field

I have a table that has 3 silly columns, they are day, month and year.
I want to join these to create a date column, I used Custom expression with concat however it comes out as text which I can expect, but how do I make it a date field.
Was thinking about using data model but not sure where to begin.
any help will be appreciated


We don’t yet cast fields in custom columns, you should be building this either in a Metabase model via sql or in a database view