Add days to input_datetime using timedelta

Configuration
2

There are several ways to do it. Yes, you can convert the input_datetime’s state to a datetime object using strptime.

{{ strptime(states('input_datetime.fridge_1_filter_changedate'), '%Y-%m-%d %H:%M:%S')
   + timedelta(days=change_days) }}

The result is a datetime object.

Alternately, you can leverage the fact an input_datetime has a timestamp attribute. We can add change_days to it (in seconds) then convert the result to local time.

{{ state_attr('input_datetime.fridge_1_filter_changedate', 'timestamp')
   + timedelta(days=change_days).seconds | timestamp_local() }}

The result is a string.