Calculate the number of days in any given year

Hello,
I am trying to determine how many days in each year so I can calculate costs based upon the number of days. I have the following columns

Commencement Date
25/2/2015
Completion date
24/2/2018
If they ask for an extension there is a further column New Scholarship extension but the value remains the same as the original date unless an extension is granted, so currently the date is.
24/2/2018

I was trying to determine how many days in 2015, how many in 2016, 2017 & 2018. I have separate cells for each of the years but cannot figure out a formula to work it out.

Many thanks,
Mandy