Populate number of days between an opportunity's close date and date service begins

Hello!

My company provides various managed services to commercial facilities. One of the things we would like to begin tracking within each opportunity record is how many days from the contract signed date (date the opportunity closes) do we begin providing services.

I have date fields created for both the contract signed date and date we begin providing services as well as a field to capture the number of days between the two fields. I thought I could use the calculated value function to subtract the start date from the closed date, but that doesn't seem to be an option. How would you recommend I go about this process?

Thank you in advance!