90 day stay

I'm wondering whether the days of arrival and departure to the island are counted as part of the 90 day stay? The reason I'm asking is that if you arrive on a Saturday and then leave 13 weeks later on a Saturday, the count of full vacation days is 90 but your arrival and departure days is a partial of 2. Would this the be ok without requiring an extension visa or not? This technicality could potentially save or add $500 US with the cab rides. Thanks in advance for all opinions.

Your arrival and departure date count as part of your 90 day stay.