Data and time math

Hey,

Bit confused on how to make this work right.

I have a date and time list and an electrical consumption list.

I want to take the date and time, subtract it from the following date and time to get the time passed. Then multiply it by the electrical consumption to get an estimate of usage.
If I just use the time then when one day goes to the next I get a negative number because I'm not taking into account the date, but I'm not sure how I put the date into my calculations.

The attached sheet is the raw data, perhaps that will be helpful to see what I'm trying to accomplish.

Thanks