Recalculation logic in billing?

Question

Algorithms

It is necessary to make a simple billing. There is a difficulty in understanding the write-off of payments.
For example : server crashed, debit periods missed.
Now the manipulation of time is on the service. Write off, change the time.
How to build logic (in 2 words) or where to look / read?

Answer 1

it is easier to bind to geo-time
to count hourly + user-friendly
user started using at 20:35 - we start counting from 21:00
the server went to bed at 21:35 - we consider that he went to bed at 21:00, etc.