Create a Absence using a Fast Formula

Hi,

I have to create a Absence to let hours to the workers to go to doctor. the limit are 8 hours and we are controlling the max hours with a fast formula. But the worker can need to modify the absence once approved and update the dates, how can I calculate the new hours dismissing the before dates in the same absence?

Please, if do you know how? you could include the code.

Thanks