Custom (independent) counters, variables and maths as sensors

I'm interested in using custom counter independent of engine hours and odometer counter.

Say for example I want to count and do math on hours worked per day (independent of engine hours).
Current way I'm sacrificing engine hour counter for my task as I need to reset it every day. And I can do this only one time.

And even so if I have to do even a bit more complex maths in notifications or sensors notifications break. ex. "sensor1-sensor2".
Even if I create SUM sensor it fails in notifications. - they fail to trigger.

Example use cases:

count up/down time worked, time in georegion, time in certain state (ex. sawmill saw on time).

Why I would want them as sensors and why stats in reports wont do?

I also want to create notifications based on these counters.

Perhaps variables should be able to bet set with "current value + x"