For those who only want it for a couple sensors, there is this simple workaround template:
value_template: '{{ ((states.sensor.the_slightly_off_sensor.state | float) -1.3) | round(1) }}'
But why should that be needed when it should be so easy to do in the code base. Since Zigbee devices love to integrate temperature sensors in seemingly every device, I ended up with over two dozen temperature sensors in my home. I may as well use this data, but when I went around with a baseline thermometer the variation is usually a few degrees and a couple are way off at like 10 degrees but consistent. My old SmartThings setup had a simple device configuration element to do an offset and I used it plenty, but HA requires a template for each one. A simple +/- item in the configuration of a temperature entity is bizarre to be lacking at this point. This would be untouched by many, but even more useless is a temperature you know is wrong. In the meantime, I guess I am changing the friendly names of the old ones and doing a mail merge to generate a chunk of code to do this in a hacky way with some extra tricks:
ball_lamp_sensor_true:
friendly_name: "Ball Lamp Temperature"
unit_of_measurement: "°F"
value_template: >-
{% if is_state('sensor.ball_lamp_air_temperature', 'unavailable') or
is_state('light.ball_lamp3', 'on') or
(as_timestamp(now()) - as_timestamp(states.sensor.ball_lamp_air_temperature.last_updated)) > 60 %}
unavailable
{% else %}
{{ (states('sensor.ball_lamp_air_temperature') | float - 1) | round(2) }}
{% endif %}
availability_template: >-
{% if is_state('sensor.front_room_sensor_temperature', 'unavailable') or
is_state('light.ball_lamp3', 'on') or
(as_timestamp(now()) - as_timestamp(is_state(sensor.living_room_sensor_air_temperature.last_updated))) > 3600 %}
unavailable
{% else %}
available
{% endif %}