Calculate time (days) between 2 Candidate Status Change

Calculate time (days) between 2 Candidate Status Change

Hi,

I am trying to create new field based on a formula which calculates the time spent between two Candidate statuses.
For example:
Candidate was Scheduled for interview on 01.01.2017
Candidate was Hired on 15.01.2017

How can I make the system calculate the days between 01.01 and 15.01?
I tried with making a custom field with a formula, but I cant find the Activity Log (where the info about status changes is kept) among the Modules available under Field Creation.

Thank you in advance for proposing any ideas.

Regards