"add" selection outputs date when passed DNP

[mdroidian]

subtract works, add doesn't.

• The Subtract column outputs an integer value when entered as subtract(Due,today)
• The Add column outputs today's date when entered as add(Due,today)

- {{query block}}
    - results
      - layout
            - mode
                - table
    - scratch
        - custom
        - selections
            - Due Date
                - Due
            - subtract(Due,today)
                - Subtract
            - add(Due,today)
                - Add
        - conditions
            - clause
                - source
                    - node
                - Relation
                    - is in page with title
                - target
                    - QB Kanban Demo Data - Attributes
            - clause
                - source
                    - node
                - Relation
                    - has attribute
                - target
                    - Project
        - return
            - node
    - editing

Reported by Chris H. via slack.