I was trying to use {{now|formatdate("%Y%m%d")}} inside a call to exec, but was getting failures indicating the entry did not have the required fields for string replacement. I narrowed it down to the use a the secrets plugin, even if the secrets file is empty. The following task demonstrates that
with secrets the output is Stdout: <function now at 0x10f9a5320> but without it you get the expected answer: Stdout: 20160224

secrets: secrets.yml
tasks:
  test2:
    manual: yes
    disable:
      - seen
      - download
      - backlog
    html:
      url: http://flexget.com/wiki/Plugins
    limit_new: 1
    accept_all: yes
    exec:
      on_output:
        for_accepted:
          - echo "{{now|formatdate("%Y%m%d")}}"
      fail_entries: true