Short answer
HubSpot's KB: calculation properties can measure the time between two date properties, or time since or until one date. The custom equation function time_between(date1, date2) returns a number. Raw Time between values are stored in milliseconds, and dates without a time use 12:00 am UTC.
1. Calculation settings
A calculation property in my demo portal.
2. Units
In a custom equation, divide milliseconds by 86,400,000 for days.