How to use sensor value in Time Trigger?

Configuration
1

Hello,
I want to trigger an automation at specified times. These times are calculated from template sensors. But I cannot find the right formatting to feed them in the time trigger:

 platform: time
 at:  "{{ (as_timestamp(states('sensor.sun_next_rising')) - (states('sensor.calculatedoffset1')|float(0)/2))|int | timestamp_custom('%H:%M:%S') }}"

The error I receive is:

Message malformed: Expected HH:MM, HH:MM:SS or Entity ID with domain ‘input_datetime’ or ‘sensor’ @ data[‘at’][0]

I tried with and without “”
If I use a template trigger where I compare the value above with output of now(function) I think I will put to much load on the HA server because it will have to run the comparison at every second.

2

Firstly please use the pre-formatted text tool, not block quotes for posting config.

Secondly, the time trigger at: option does not support templates. Only HH:MM:SS or a sensor or input number entity that contains a time in that format. e.g.

trigger:
  - platform: time
    at: input_number.foobar # <-- not a template

Look at the other examples here: https://www.home-assistant.io/docs/automation/trigger/#time-trigger no templates.

Use a template trigger.