How to use datetime_add in set_attribute?

change set_attribute(:trial_ends_at, expr(datetime_add(trial_ends_at, 7, "day")))

%Ash.Error.Invalid{changeset: "#Changeset<>",  errors: [%Ash.Error.Changes.InvalidAttribute{field: :trial_ends_at, message: "Could not cast input to datetime", private_vars: nil, value: datetime_add(trial_ends_at,  7,  :day), splode: Ash.Error, bread_crumbs: [], vars: [], path: [], stacktrace: #Splode.Stacktrace<>, class: :invalid}]}