What Constitutes Leasee's 30 day notice to vacate? (FL) [xpost /r/PropertyManagement]

I am renting a home and needed to give a 30 day notice to vacate at the termination of my lease (in Florida). I sent it 2 days ago (4/12/16) by certified mail as stipulated in the lease. The lease ends on 5/13/16. This is 30 days exactly counting the day I sent it (4/12) and 5/13.

A few questions. 1. Does the date it is postmarked count as day 1? 2. Does the last day of the lease count as day 30? 3. If the landlord holds my feet to the fire and tries to charge another 30 days of rent (the lease states tenancy will resume as month to month with the full month's amount due w/o 30 day notice), will that hold up?

Any help will be appreciated.