There are 2 main concepts to keep in mind when is about to extract part of a variable containing a date.

+ the variable containing the date need to be a scale variable (S), if it's a text variable the function applied does not work.

+ the functions to use are not pure VBscript, but VBscript alike, so in this specific case the available functions are date_year(varname), date_month(varname) and date_date(varname).

A list of the full VBscript alike functions to use in the formula field is available here