How to filter with datetime?

I would like to retrieve items only from last month and therefore tried to create a filter that looks the following: SUBTRACT_DURATION(NOW(""), 3, "months")
This is stored in a page variable with type date/time text and used in the filter criteria.

Unfortunately this still seems to convert my DateTime into a string. CAP does not like this and throws a bad request with the following error:
UriQueryOptionSemanticError: The type ‘Edm.DateTimeOffset’ is not compatible to ‘Edm.String’

Do you know how to get around this?