Number of days to Full Moon this Month Automation

Configuration
7

I have tried making a template for this, but no luck so far this is what I have, but but it reports Unavailable

sensor:
  - platform: template
    sensors:
      days_until_full_moon:
        friendly_name: "Days Until Full Moon"
        unit_of_measurement: 'days'
        value_template: >
          {% set today = now().date() %}
          {% set next_full_moon = states('sensor.moon') | as_datetime %}
          {% set full_moon_date = next_full_moon.date() %}
          {% set days_until_full_moon = (full_moon_date - today).days %}
          {{ days_until_full_moon }}