Calendar dates or an exact moment?
Use age or date difference for years, months, and days. Use days between dates for a whole-day count. A calendar month has no fixed number of days, so dividing a day count by 30 will not reproduce a calendar difference.
Count the endpoints deliberately
From September 1 to September 2 is one elapsed day. An inclusive date range contains two dates. Check the endpoint option when counting booked days or an event period.
For meetings and system logs
A time-zone conversion needs the date as well as the time because daylight-saving offsets change. A Unix timestamp represents an instant; check whether your source uses seconds or milliseconds before converting.