Import Statistics - Can I back fill data?

[gedger]

I have started to calculate degree days for my location to be able to analyse my heating (https://www.degreedays.net). As you need the daily average temperature to calculate the degree day it's not possible until 23:59:59 to do the calculation. So at 23:59:59 you now have the calculate "degree day" for the day that has just passed. Storing the value at 23:59:59 in HA results in the degree day being offset by 1 day, i.e. it's a day late.

So my idea was to use the Import Statistics option of Spook to import the value calculated at 23:59:59 with the start time of 00:00:00 on the same day thus removing the offset. I can't make this work, what am I doing wrong?

service: recorder.import_statistics
data:
  has_mean: false
  has_sum: false
  statistic_id: sensor.test_degree_day_daily
  source: recorder
  name: "NULL"
  unit_of_measurement: °C
  stats:
    - start: "{{ now().replace(hour=0, minute=0, second=0, microsecond=0) }}"
      state: >
        {% set regularized_temp = 18.0 %}

        {% set average_outside_temp =
        states('sensor.daily_average_outdoor_temperature') | float(10) %}

        {% set dd = regularized_temp - average_outside_temp %}

        {% if dd > 0 %}
          {{ dd }}
        {% else %}
          0
        {% endif %}