Hi can you confirm that CB subs can be configured with calendar based subscription, so I can set my yearly membersuip to start from jan 1 to dec 31?
Yes, there is by calendar based subscription durations for year and month.
Is it possible so set a "tolerance period", to consider membership still valid after expiration? (something like "Your membership's expired, you can renew within 30 days")
Yes, you can configure a Grace period before actual expiration.
Is it possible to set an "advance registration", so a user registering in december will be considered a member for the following year?
Yes, the by calendar durations let you specify a "bonus" time allowing users that renew for example in the last 3 months (this is configurable of course.. could be 1 month for example) of the year to get those 3 months free and they're basically subscribing for the following year.