Using Firebird 2.0, how can I format a date to yyyymmdd?

For example Jan 15 2009 -> 20090115

My first approach has been using EXTRACT(YEAR FROM BirthDate) || EXTRACT(MONTH FROM BirthDate) || EXTRACT(DAY FROM BirthDate)
but it does not work for me because I need to lead zeros and I cannot find how do it. Maybe there is a direct function.

Thanks