The current date and time is determined section, and it shows you when you want to set the date whether some specific month or day is leaping or not during that year.
In addition, if you have the Clock enabled, it will also be displayed here.
The Preview Date is a way of previewing past dates, without actually setting the current date. This is useful if you need to go to a future date to set up an event or go into the past to see when the last full moon was.
The Preview Date is enabled by default for guests viewing your calendar. You can disable that privilege entirely in the settings under "Enable previewing dates in calendar", or disable viewing future dates with "Limit previewing to only past dates". These settings do not affect you, the owner.