My apologies; looks like my suggestion was wrong. It’s reporting a running sum of the temperature values (which is not what you want).
I suggest you consider doing what this person did:
The user provides several examples for resetting at various time intervals (except for yearly). Here’s my suggestion for resetting on January 1st at 00:00.
value_template: "{{ now().month == 1 and now().day == 1 and now().hour == 0 and now().minute == 0 }}"
It’s reporting a running sum of the temperature values (which is not what you want).