I’ve created a similar scenario actually where Start Date is a Date column that is manually set by the user.
Then I created a Term column (e.g. 5 years) using the Number column. So that shows as 5 Years.
Then I created a Days column using the Formula column “Multiply ({term}, 365)”
Finally, to create an Expiration date, I used the Formula column again "Format Date (ADD DAYS ({effective date}, {DAYS}), “MM-DD-YYYY”)
You may be able to use the Formula column to do something like, Count Days “Start Date” to “End Date” to arrive at your number of days. There’s also an ‘Item Age’ Column type you may be able to modify to show only Days vs minutes and hours.