The tax year runs from January 1st to December 31st. In Florida, property taxes are paid in arrears. The tax roll is certified by the Property Appraiser in October.
If closing is Monday 3/21/20011 with the day of closing being charged to the Buyer the Seller is debited for 79 days and this becomes a credit to the Buyer.
Remember taxes for the year have not been paid and the Seller occupied the property the property 31 days in January, 28 days in February and 20 days in March so they owe the buyer.
Hope this helps,